Sounds like an obvious one, but all too often people get blamed for damage they didn't cause. This usually occurs because the employee is a rush to get the car out and doesn't have time to inspect the car with you. Remember: you are the customer and no matter how busy and tired they are, you should insist that they take the time to note all the marks and scratches with you before you sign. Petrol is another area that can cause unnecessary expense. You can pay a ridiculous premium per gallon if the petrol gauge is not the same as when you started. Always ask for the tank to be full.