One day car insurance, just like every other form of motor insurance, has restrictions. Whilst cover is described as 'comprehensive' it cannot indemnify you against every possible risk without the premiums being stratospherically high so here is a list of common ways in which you would not be covered; these vary of course from one company to another so you should check the policies carefully before buying insurance to make sure that they meet your needs.
1) You would be expected to take reasonable care of the insured vehicle. Leaving it with a window open, leaving it running on your driveway to warm it up or melt ice off a windscreen, leaving the keys in the ignition whilst you pay at a filling station, all these could result in the car or it's contents being stolen; and the insurance company would probably, quite reasonably, refuse to pay out.
2) Put the wrong fuel in, or suffer frost damage because you have not put in insufficient antifreeze, and the insurer would no doubt be sympathetic would not pay for the damage.3) There will probably be a limit on the amount of any payout for damages to property, and if you damage any of your own property it will probably not be covered.
4) You will be covered for driving the insured vehicle only under your short term policy, and not for any other vehicles, whether you own them or not.
5) You could not use a short-term policy to help you recover a vehicle which had been taken to a police pound.
6) You would have to be sensible about where you took the vehicle. Driving it on unmade roads or to other hazardous locations could invalidate the policy.
These are just a few of the more common restrictions; since there are a number of insurers that may issue you with a policy you should check their documentation individually to see what they cover, and more importantly what they do not.