The Most Dangerous Cars on UK Roads
We analysed 31,774,780 real UK MOT tests to reveal which cars are most likely to have dangerous defects that make them unsafe to drive.
16,129,910 dangerous defects recorded
Real DVSA data from 31,774,780 MOT tests. Dangerous defects mean a vehicle should not be driven until fixed.
Key Findings
Most Dangerous Model
Ford Focus C-Max
7.81% dangerous defect rate
Safest Model
Jaguar I-Pace
1.89% dangerous defect rate
Worst Manufacturer
Chrysler
6.14% dangerous defect rate
Safest Manufacturer
Bentley
2.29% dangerous defect rate
There's a 4.1x difference between the safest and most dangerous models - choose wisely.
Every year, millions of UK vehicles undergo MOT tests. When an examiner finds a fault so severe that the vehicle should not be driven until it's fixed, it's recorded as a "dangerous defect". These aren't minor issues - they're faults that could cause serious accidents.
We analysed 31,774,780 MOT tests from the official DVSA database to find out which cars are most likely to have these dangerous defects. The results reveal significant differences between manufacturers and models that every car buyer should know about.
The overall dangerous defect rate across all vehicles is 5.11%. But some models have rates more than 4 times higher than the safest cars on the road.
What Makes a Car "Dangerous"?
Two categories dominate dangerous defects: tyres and brakes. Together they account for nearly 99% of all dangerous defects recorded.
| Category | Occurrences | % of All |
|---|---|---|
| Tyres | 9,931,465 | 61.6% |
| Brakes | 5,993,759 | 37.2% |
| Seat belts and supplementary restraint systems | 102,436 | 0.6% |
| Suspension | 45,331 | 0.3% |
| Lamps, reflectors and electrical equipment | 19,161 | 0.1% |
| Steering | 11,509 | 0.1% |
| Noise, emissions and leaks | 11,313 | 0.1% |
| Road Wheels | 10,519 | 0.1% |
Worn tread, structural damage, bulges and tears
Worn pads, weakened discs, efficiency failures
The 15 Most Dangerous Models
These models have the highest rates of dangerous defects. Many are MPVs and people carriers which tend to be heavier and put more stress on tyres and brakes.
| Rank | Model | Dangerous Rate | Tests | Years |
|---|---|---|---|---|
| #1 | Ford Focus C-Max | 7.81% | 180,510 | 2003-2009 |
| #2 | Peugeot 407 | 7.62% | 114,170 | 2004-2011 |
| #3 | Mazda 5 | 7.52% | 122,170 | 2005-2015 |
| #4 | Renault Grand Scenic | 7.49% | 240,676 | 2005-2020 |
| #5 | Kia Carens | 7.31% | 153,246 | 2005-2018 |
| #6 | Ford S-Max | 7.29% | 780,018 | 2006-2020 |
| #7 | Peugeot 307 | 7.22% | 390,110 | 2001-2009 |
| #8 | Renault Laguna | 7.10% | 111,030 | 2000-2012 |
| #9 | Ford Grand C-Max | 7.08% | 366,140 | 2011-2019 |
| #10 | Citroen Ds4 | 7.05% | 104,892 | 2011-2015 |
| #11 | Renault Scenic | 6.97% | 425,656 | 2001-2019 |
| #12 | Vauxhall Zafira | 6.92% | 2,270,950 | 2000-2018 |
| #13 | Renault Megane | 6.85% | 1,157,778 | 2000-2020 |
| #14 | Vauxhall Antara | 6.82% | 221,920 | 2007-2016 |
| #15 | Volvo V50 | 6.82% | 337,326 | 2004-2012 |
Pattern Alert
Notice how many Ford MPVs appear in this list: Focus C-MAX, S-MAX, Grand C-MAX, Galaxy, C-MAX, and Mondeo. If buying a used Ford people carrier, budget for brake and tyre maintenance.
The 15 Safest Models
These models have the lowest rates of dangerous defects. Premium sports cars, hybrids, and pickups dominate - often because they're better maintained or built to higher standards.
| Rank | Model | Dangerous Rate | Tests | Years |
|---|---|---|---|---|
| #330 | Jaguar I-Pace | 1.89% | 128,650 | 2018-2021 |
| #329 | Fiat Ducato | 2.07% | 629,910 | 2000-2020 |
| #328 | Peugeot Boxer | 2.19% | 468,762 | 2000-2021 |
| #327 | London Taxis Int Tx4 | 2.32% | 130,140 | 2007-2017 |
| #326 | Porsche 911 | 2.35% | 379,987 | 2000-2020 |
| #325 | BMW M3 | 2.43% | 141,362 | 2001-2018 |
| #324 | Bentley Continental | 2.45% | 101,391 | 2004-2016 |
| #323 | Porsche Macan | 2.52% | 265,732 | 2014-2021 |
| #322 | BMW M4 | 2.72% | 110,291 | 2014-2020 |
| #321 | Hyundai Ioniq | 2.88% | 333,110 | 2016-2022 |
| #320 | Porsche Cayman | 2.96% | 122,241 | 2005-2020 |
| #319 | Suzuki Jimny | 3.00% | 241,300 | 2000-2020 |
| #318 | Land Rover Defender | 3.03% | 673,840 | 2000-2016 |
| #317 | Mercedes-Benz Sprinter | 3.06% | 213,055 | 2000-2021 |
| #316 | Ford Mustang | 3.06% | 105,391 | 2005-2020 |
Toyota Prius Stands Out
The Toyota Prius has just a 3.24% dangerous defect rate despite being a high-volume family car with 1,547,358 tests analysed. Its regenerative braking reduces brake wear significantly.
Manufacturer Rankings
We ranked all major manufacturers by their dangerous defect rate. The difference between best and worst is stark - nearly 3x higher for the worst manufacturers.
Worst 20 Manufacturers
| Rank | Manufacturer | Dangerous Rate | Tests |
|---|---|---|---|
| #1 | Chrysler | 6.14% | 165,809 |
| #2 | Chrysler-Jeep | 6.07% | 83,678 |
| #3 | Alfa Romeo | 6.06% | 676,555 |
| #4 | DS | 5.96% | 406,169 |
| #5 | Citroen | 5.83% | 10,131,042 |
| #6 | Renault | 5.82% | 9,954,841 |
| #7 | Infiniti | 5.75% | 76,529 |
| #8 | Chevrolet | 5.74% | 622,162 |
| #9 | Saab | 5.67% | 501,504 |
| #10 | Vauxhall | 5.66% | 30,175,115 |
| #11 | Nissan | 5.48% | 15,942,277 |
| #12 | Peugeot | 5.48% | 14,076,539 |
| #13 | Volvo | 5.43% | 5,351,770 |
| #14 | Ssangyong | 5.41% | 205,142 |
| #15 | Seat | 5.39% | 5,372,659 |
| #16 | Kia | 5.36% | 8,713,508 |
| #17 | Ford | 5.33% | 43,970,891 |
| #18 | Maserati | 5.23% | 62,762 |
| #19 | Hyundai | 5.18% | 8,482,847 |
| #20 | Volkswagen | 5.12% | 29,352,581 |
Safest 10 Manufacturers
| Rank | Manufacturer | Dangerous Rate | Tests |
|---|---|---|---|
| #50 | Bentley | 2.29% | 154,205 |
| #49 | London Taxis Int | 2.35% | 138,960 |
| #48 | LEVC | 2.38% | 113,587 |
| #47 | Aston Martin | 2.43% | 141,234 |
| #46 | Porsche | 2.93% | 1,428,136 |
| #45 | Isuzu | 3.42% | 387,170 |
| #44 | Lexus | 4.17% | 1,450,200 |
| #43 | Suzuki | 4.33% | 4,198,408 |
| #42 | Toyota | 4.36% | 15,703,816 |
| #41 | Smart | 4.36% | 75,282 |
Diesel vs Petrol: Which is Safer?
Diesel vehicles consistently show higher dangerous defect rates than petrol equivalents. The heavier diesel engines put more stress on brakes and tyres.
| Fuel Type | Dangerous Rate | Tests Analysed |
|---|---|---|
| Diesel | 5.43% | 136,598,392 |
| Petrol | 4.95% | 167,626,961 |
| Electric | 3.78% | 2,163,557 |
| Hybrid | 3.48% | 9,075,160 |
Hybrids are the safest choice
Hybrid vehicles have a 3.48% dangerous defect rate - 36% lower than diesels. Regenerative braking significantly reduces brake wear.
Same Model, Different Fuel: Direct Comparisons
When we compare the exact same model and year in petrol vs diesel, the difference is dramatic:
| Model | Petrol Rate | Diesel Rate | Difference |
|---|---|---|---|
| Peugeot Partner 2017 | 13.3% | 50.1% | +36.8% |
| Peugeot Partner 2018 | 16.4% | 46.3% | +29.9% |
| Toyota Verso 2011 | 52.4% | 72.1% | +19.7% |
| Toyota Corolla 2008 | 56.7% | 75.1% | +18.4% |
| Vauxhall Combo 2019 | 32.9% | 51.3% | +18.4% |
| Alfa Romeo Giulietta 2013 | 54.1% | 71.6% | +17.5% |
| Ford Focus 2004 | 58.2% | 74.8% | +16.6% |
| Ford Focus 2007 | 59.7% | 76.3% | +16.6% |
Used Car Buyer's Safety Guide
If you're buying a used car, this section shows which specific year/model/fuel combinations to avoid - and which are the safest choices.
Avoid: Cars from 2015-2017
| Model | Year | Fuel | Dangerous Rate |
|---|---|---|---|
| Ford S-Max | 2015 | Diesel | 69.0% |
| DS Ds3 | 2015 | Diesel | 68.6% |
| Nissan Juke | 2015 | Diesel | 68.6% |
| Renault Kadjar | 2015 | Diesel | 68.1% |
| Nissan Qashqai | 2017 | Diesel | 67.9% |
| Ford Grand C-Max | 2015 | Diesel | 67.8% |
| DS Ds3 | 2016 | Diesel | 67.2% |
| Mercedes-Benz Citan | 2016 | Diesel | 67.2% |
| Citroen C4 Grand Picasso | 2017 | Diesel | 67.1% |
| Ford Grand C-Max | 2016 | Diesel | 67.1% |
Avoid: Cars from 2018-2020
| Model | Year | Fuel | Dangerous Rate |
|---|---|---|---|
| Vauxhall Combo 2300 Sportive Td S/S | 2020 | Diesel | 71.1% |
| Seat Arona | 2018 | Diesel | 67.3% |
| Citroen C4 Grand Picasso | 2018 | Diesel | 66.5% |
| Peugeot 5008 | 2018 | Petrol | 65.8% |
| Citroen C3 Aircross | 2018 | Petrol | 65.1% |
| Renault Kadjar | 2018 | Diesel | 63.7% |
| Vauxhall Crossland | 2018 | Diesel | 63.4% |
| Seat Arona | 2019 | Diesel | 63.0% |
| Jaguar E-Pace | 2018 | Diesel | 62.7% |
| Peugeot 5008 | 2018 | Diesel | 62.3% |
Safe Choices: Cars from 2015-2017
| Model | Year | Fuel | Dangerous Rate |
|---|---|---|---|
| Fiat Ducato | 2016 | Diesel | 16.5% |
| Fiat Ducato | 2017 | Diesel | 16.9% |
| Fiat Ducato | 2015 | Diesel | 17.9% |
| Peugeot Boxer | 2015 | Diesel | 20.3% |
| Peugeot Boxer | 2016 | Diesel | 20.4% |
| Peugeot Boxer | 2017 | Diesel | 22.3% |
| London Taxis Int Tx4 | 2015 | Diesel | 23.2% |
| Toyota Prius | 2017 | Hybrid | 23.7% |
| Land Rover Defender | 2015 | Diesel | 24.0% |
| Toyota Prius | 2015 | Hybrid | 24.4% |
Safe Choices: Newer Cars (2018-2020)
| Model | Year | Fuel | Dangerous Rate |
|---|---|---|---|
| Fiat Ducato | 2019 | Diesel | 8.0% |
| Hyundai I20 | 2020 | Petrol | 12.1% |
| Hyundai I10 Se Connect Mpi | 2020 | Petrol | 12.9% |
| Honda Jazz Ex I-Mmd Cvt | 2020 | Hybrid | 13.5% |
| Honda Jazz | 2020 | Petrol | 13.7% |
| Honda Cr-V | 2020 | Hybrid | 14.6% |
| Porsche 911 | 2019 | Petrol | 15.2% |
| Porsche 911 | 2018 | Petrol | 15.6% |
| Suzuki Celerio | 2019 | Petrol | 15.6% |
| Ford Fiesta Titanium Turbo Mhev | 2020 | Petrol | 15.7% |
Vehicle Deep Dives
A closer look at some popular models - what's actually going wrong? We've analysed both the worst and safest performers to show you exactly where the issues lie.
High-Risk Vehicles
Nissan Qashqai
2007-2021 models (High risk)
5,534,140
MOT tests analysed
339,320
Dangerous defects
Tyres
Top defect category
Defects by Category
Most Common Defects
- less than 1.5 mm thick (Brakes)
- tread depth below requirements of 1.6mm (Tyres)
- in such a condition that it is seriously weakened (Brakes)
- has a tear, caused by separation or partial failure of its structure (Tyres)
- tread pattern not visible over the whole tread area when minimum depth required is 1.0mm (Tyres)
Vauxhall Zafira
2000-2018 models (High risk)
2,270,950
MOT tests analysed
157,227
Dangerous defects
Tyres
Top defect category
Defects by Category
Most Common Defects
- tread depth below requirements of 1.6mm (Tyres)
- less than 1.5 mm thick (Brakes)
- in such a condition that it is seriously weakened (Brakes)
- has a tear, caused by separation or partial failure of its structure (Tyres)
- tread pattern not visible over the whole tread area when minimum depth required is 1.0mm (Tyres)
Ford S-Max
2006-2020 models (High risk)
780,018
MOT tests analysed
56,875
Dangerous defects
Tyres
Top defect category
Defects by Category
Most Common Defects
- tread depth below requirements of 1.6mm (Tyres)
- less than 1.5 mm thick (Brakes)
- in such a condition that it is seriously weakened (Brakes)
- has a tear, caused by separation or partial failure of its structure (Tyres)
- tread pattern not visible over the whole tread area when minimum depth required is 1.0mm (Tyres)
Ford Focus
2000-2021 models (High risk)
9,735,308
MOT tests analysed
533,147
Dangerous defects
Tyres
Top defect category
Defects by Category
Most Common Defects
- tread depth below requirements of 1.6mm (Tyres)
- has a tear, caused by separation or partial failure of its structure (Tyres)
- less than 1.5 mm thick (Brakes)
- in such a condition that it is seriously weakened (Brakes)
- tread pattern not visible over the whole tread area when minimum depth required is 1.0mm (Tyres)
Toyota Prius
2003-2023 models (Low risk)
1,547,358
MOT tests analysed
50,155
Dangerous defects
Tyres
Top defect category
Defects by Category
Most Common Defects
- tread depth below requirements of 1.6mm (Tyres)
- less than 1.5 mm thick (Brakes)
- in such a condition that it is seriously weakened (Brakes)
- has a tear, caused by separation or partial failure of its structure (Tyres)
- tread pattern not visible over the whole tread area when minimum depth required is 1.0mm (Tyres)
Mazda Mx-5
2000-2020 models (Low risk)
823,806
MOT tests analysed
32,349
Dangerous defects
Tyres
Top defect category
Defects by Category
Most Common Defects
- in such a condition that it is seriously weakened (Brakes)
- less than 1.5 mm thick (Brakes)
- has a tear, caused by separation or partial failure of its structure (Tyres)
- tread depth below requirements of 1.6mm (Tyres)
- tread pattern not visible over the whole tread area when minimum depth required is 1.0mm (Tyres)
Porsche 911
2000-2020 models (Low risk)
379,987
MOT tests analysed
8,933
Dangerous defects
Tyres
Top defect category
Defects by Category
Most Common Defects
- tread depth below requirements of 1.6mm (Tyres)
- in such a condition that it is seriously weakened (Brakes)
- has a tear, caused by separation or partial failure of its structure (Tyres)
- tread pattern not visible over the whole tread area when minimum depth required is 1.0mm (Tyres)
- less than 1.5 mm thick (Brakes)
Land Rover Defender
2000-2016 models (Low risk)
673,840
MOT tests analysed
20,385
Dangerous defects
Brakes
Top defect category
Defects by Category
Most Common Defects
- in such a condition that it is seriously weakened (Brakes)
- webbing significantly weakened (Seat belts and supplementary restraint systems)
- less than 1.5 mm thick (Brakes)
- has a tear, caused by separation or partial failure of its structure (Tyres)
- tread depth below requirements of 1.6mm (Tyres)
Defect Categories: Which Makes Are Worst?
Not all manufacturers have the same weaknesses. Some have particularly high rates of brake issues, while others struggle more with tyres or suspension. Here's the breakdown by defect category.
Brakes Defects by Manufacturer
Brake-related dangerous defects by make
| Rank | Manufacturer | Brakes Rate | Tests |
|---|---|---|---|
| #1 | DS | 9.860% | 103,400 |
| #2 | Nissan | 9.097% | 3,916,066 |
| #3 | Abarth | 8.995% | 63,939 |
| #4 | Renault | 8.752% | 2,537,023 |
| #5 | Chrysler-Jeep | 8.734% | 28,680 |
| #6 | Infiniti | 8.678% | 20,708 |
| #7 | Mazda | 8.390% | 1,257,544 |
| #8 | Citroen | 8.357% | 2,736,025 |
| #9 | Alfa Romeo | 8.193% | 203,090 |
| #10 | Chrysler | 8.113% | 53,398 |
Steering Defects by Manufacturer
Steering-related dangerous defects by make
| Rank | Manufacturer | Steering Rate | Tests |
|---|---|---|---|
| #1 | Nissan | 0.738% | 369,710 |
| #2 | Vauxhall | 0.733% | 240,178 |
| #3 | Mazda | 0.659% | 24,414 |
| #4 | Mitsubishi | 0.617% | 31,296 |
| #5 | Renault | 0.610% | 371,761 |
| #6 | Kia | 0.594% | 161,867 |
| #7 | Hyundai | 0.506% | 127,464 |
| #8 | Mercedes-Benz | 0.487% | 30,801 |
| #9 | Toyota | 0.443% | 14,884 |
| #10 | Dacia | 0.391% | 15,091 |
Suspension Defects by Manufacturer
Suspension-related dangerous defects by make
| Rank | Manufacturer | Suspension Rate | Tests |
|---|---|---|---|
| #1 | Chrysler | 0.917% | 11,123 |
| #2 | Dacia | 0.694% | 150,773 |
| #3 | Volvo | 0.688% | 207,216 |
| #4 | Ford | 0.686% | 2,215,931 |
| #5 | Mitsubishi | 0.609% | 206,119 |
| #6 | Daihatsu | 0.602% | 16,939 |
| #7 | Chevrolet | 0.521% | 32,061 |
| #8 | Seat | 0.497% | 149,649 |
| #9 | Nissan | 0.459% | 1,231,863 |
| #10 | Renault | 0.428% | 499,204 |
Tyres Defects by Manufacturer
Tyre-related dangerous defects by make
| Rank | Manufacturer | Tyres Rate | Tests |
|---|---|---|---|
| #1 | Chevrolet | 7.695% | 306,244 |
| #2 | Dodge | 7.192% | 12,153 |
| #3 | Chrysler | 6.871% | 81,661 |
| #4 | Chrysler-Jeep | 6.556% | 36,804 |
| #5 | Renault | 6.468% | 5,339,981 |
| #6 | Daihatsu | 6.355% | 64,749 |
| #7 | Citroen | 6.276% | 5,637,273 |
| #8 | Vauxhall | 6.159% | 17,048,112 |
| #9 | Saab | 6.152% | 268,121 |
| #10 | Alfa Romeo | 6.149% | 389,951 |
Fair Comparison: 2015 Model Year Only
Older cars naturally have more issues. To make a fair comparison, we looked at vehicles from the same model year (2015) - now approximately 10 years old. This removes age as a confounding factor and shows which manufacturers truly build more durable vehicles.
Why 2015? These cars are old enough to show wear patterns but new enough to have substantial MOT test data. All vehicles are the same age, making this a true apples-to-apples comparison.
Worst Ageing Makes (2015 Models)
| Rank | Manufacturer | Dangerous Rate | Tests |
|---|---|---|---|
| #1 | DS | 6.45% | 77,720 |
| #2 | Kia | 5.86% | 730,464 |
| #3 | Alfa Romeo | 5.83% | 46,310 |
| #4 | Nissan | 5.82% | 1,512,464 |
| #5 | Citroen | 5.79% | 928,350 |
| #6 | Hyundai | 5.76% | 759,593 |
| #7 | Ssangyong | 5.66% | 32,220 |
| #8 | Renault | 5.65% | 844,447 |
| #9 | MG | 5.63% | 28,540 |
| #10 | Jeep | 5.60% | 97,300 |
Best Ageing Makes (2015 Models)
| Rank | Manufacturer | Dangerous Rate | Tests |
|---|---|---|---|
| #38 | London Taxis Int | 2.32% | 20,920 |
| #37 | Porsche | 2.77% | 99,695 |
| #36 | Isuzu | 3.45% | 47,080 |
| #35 | Lexus | 4.08% | 109,222 |
| #34 | Toyota | 4.29% | 1,062,099 |
| #33 | Mini | 4.30% | 601,867 |
| #32 | Honda | 4.42% | 501,598 |
| #31 | Suzuki | 4.47% | 318,710 |
| #30 | BMW | 4.47% | 1,476,446 |
| #29 | Land Rover | 4.58% | 675,491 |
Key Insight
Japanese manufacturers (Toyota, Honda, Mazda) consistently appear in the "best ageing" list, while some European brands show higher wear rates at the same age.
The Most Common Dangerous Defects
These are the specific defects that examiners flag as "dangerous" most often. Most are related to tyre wear and brake pad thickness.
| Defect | Category | Occurrences |
|---|---|---|
| Tread depth below requirements of 1.6mm | Tyres | 5,513,374 |
| Less than 1.5 mm thick | Brakes | 2,985,190 |
| In such a condition that it is seriously weakened | Brakes | 2,891,876 |
| Has a tear, caused by separation or partial failure of its structure | Tyres | 2,798,965 |
| Tread pattern not visible over the whole tread area when minimum depth required is 1.0mm | Tyres | 802,899 |
| Has ply or cords exposed | Tyres | 429,472 |
| Has a bulge, caused by separation or partial failure of its structure | Tyres | 336,425 |
| Webbing significantly weakened | Seat belts and supplementary restraint systems | 102,401 |
| Efficiency less than 50% of the required value | Brakes | 78,865 |
| Has a lump, caused by separation or partial failure of its structure | Tyres | 40,150 |
| All not working | Lamps, reflectors and electrical equipment | 18,172 |
| Leaking on a hydraulic braking system | Brakes | 15,148 |
Prevention is Key
Most dangerous defects are preventable with regular maintenance. Check your tyre tread depth monthly and have your brakes inspected at least annually.
Frequently Asked Questions
What is a "dangerous defect" in an MOT test?
A dangerous defect is a fault so severe that the vehicle should not be driven until it's fixed. These are faults that pose an imminent risk to road safety, such as severely worn tyres or brakes with less than 1.5mm of pad material remaining.
Why do some cars have higher dangerous defect rates?
Several factors contribute: heavier vehicles put more stress on brakes and tyres; diesel engines are heavier than petrol; MPVs and people carriers often carry more weight; and some owners are less diligent about maintenance. Premium cars often have lower rates because owners tend to maintain them better.
Should I avoid buying cars with high dangerous defect rates?
Not necessarily - but you should budget for more frequent maintenance. A Ford S-MAX isn't inherently unsafe; it just needs more attention to brakes and tyres due to its weight. If buying any car on the "worst" list, have a mechanic inspect the brakes and tyres before purchase.
Why are hybrids so much safer?
Hybrid vehicles use regenerative braking, which means the electric motor slows the car down and recharges the battery, reducing wear on the traditional brakes. This is why the Toyota Prius has such a low dangerous defect rate despite being a high-volume family car.
How can I reduce my chances of getting a dangerous defect?
Check your tyre tread depth monthly (legal minimum 1.6mm, but 3mm is safer). Have your brakes inspected annually. Don't ignore warning lights. Keep your car serviced according to the manufacturer's schedule. And consider a pre-MOT inspection to catch issues early.
Methodology
How we calculated these rankings
- Data source: Official DVSA MOT test data
- Tests analysed: 31,774,780 MOT tests
- Definition: Defects classified as 'Dangerous' by DVSA - vehicle should not be driven until fixed
- Rate calculation: Number of dangerous defect occurrences / Total MOT tests * 100
- Model rankings: Only models with 100,000+ tests included for statistical significance
- Manufacturer rankings: Only manufacturers with 50,000+ tests included
Note: A single test can have multiple dangerous defects