You're driving below the speed limit and obeying the traffic signals when another driver plows into your car. You might assume the other guy's insurance company should pay your medical and car repair bills -- but that's not the case in every instance. "Unfortunately it's not as simple as that," says Insure.com consumer analyst Penny Gusner. "The little details of the accident really do matter." For the other driver to be liable for the accident, and for his liability insurance to kick in, there must be evidence that he was negligent. "Liability insurance is only going to pay if the injured party can prove the other driver is at fault in court or out of court," says attorney Benjamin Zimmermann, a partner with Sugarman & Sugarman P.C. in Boston. "If you can't prove negligence, you can't win the case, and if you can't win the case, insurance companies know that and won't pay. One of the keys to a successful claim is to establish the other driver's fault early and thoroughly." The rules also vary by state, notes David Reischer, a New York attorney and co-founder of LegalAdvice.com. In states with no-fault auto insurance systems, your own insurance generally pays for your medical bills, regardless of who was at fault, and you're restricted in when you can sue other drivers for injuries. However, in most no-fault states an at-fault driver may still be liable for property damage. It's a good idea to understand how the rules work where you live before you have an accident because insurance laws vary widely by state. If you don't have collision coverage, which would pay for repairs to your car in an accident, you have to rely on the at-fault driver's insurance to pay for repairs.Here are five scenarios when the other driver's insurance company may refuse to pay out -- even if you think it should.