Our flight to Málaga, Spain, arrived late at night. We were exhausted, so we picked up our rental car quickly and drove to the hotel. Little did we know what an expensive mistake that was. The next morning, we noticed a three-inch scratch on the front bumper of our car. We had parked head-in and away from other cars in the lot, so it must have been there when we picked up the rental. We didn't think of it again.
Upon returning the car a week later, we were told the front bumper was damaged and we'd be charged for the repair. We tried to explain that it likely had been there when we picked up the rental (and that it wasn't even that big a scratch). No matter. Our credit card was charged $200 to fix the damage - too low an amount for our personal car insurance to kick in.
We had learned an expensive lesson. Always document the condition of your car when you pick it up. The rental car company prepares a form showing a black outline of a car, which a staffer marks with Xs to show any existing scratches or dents.
In our experience, you should examine both the form and the vehicle carefully. Walk around the car and add to the form any marks you see, no matter how small. We also follow our friend Paula's recommendation and take photos of any existing damage and scratches - with the car rental lot visible in the background to show we haven't yet begun to drive the car. This may sound paranoid. But $200 later, we aren't taking any chances.