Seemingly innocent slip-ups, such as wearing heels while you are driving, are causing £114 million in rejected car insurance claims a year.
Other slip-ups include drivers forgetting to renew their road tax and even wearing the wrong footwear.
That's according to new research by Uswitch, which revealed that two-thirds of drivers are putting their future payouts at risk by making simple - and frivolous - mistakes that could void their policy out entirely.
In a list of the most common misdemeanors made by motorists, the comparison website revealed wearing heels or flip flops as the easiest way to get your insurance written off.
A quarter of drivers sampled by Uswitch said they've worn this type of footwear while driving.
The report also found many motorists were unaware that items hanging from their windscreen or rear-view mirror that obstruct their view could invalidate their cover.
The survey of 1,593 consumers found the most forgetful drivers are dotted around Birmingham, where one in three people on the road routinely forget to lock their cars.
Simple errors, which also involve forgetting to strap in your pet, could mean that British drivers lose out on £114 million each year in unpaid claims based on an average claim amount of £2,671, according to Association of British Insurers' research.
A quarter of drivers admitted they did not realise these small errors could result in car insurance providers not paying out, with an additional third of drivers admitting they simply forget that such behaviours might impact on an insurance claim.
The latest data from the ABI shows that over 43,000 car insurance claims are rejected a year.
10 slip-ups that can invalidate your insurance
- Wearing high heels or flip flops while driving - 25 per cent of drivers.
- Leaving car unlocked - 24 per cent.
- Putting off car maintenance when required - 21 per cent.
- Forgetting to renew their car MOT on time - 16 per cent.
- Letting pet(s) roam free in the car - 15 per cent.
- Lent car to a friend or family member - 14 per cent.
- Not updated details after changing jobs - nine per cent
- Attached fluffy dice, or another object, to rear-view mirror - nine per cent
- Forgetting to renew tax - 9%
- Underestimated mileage travelled everyday - 8%