Rental car companies in Europe charge about $15 per gallon if the car is returned without a full tank. One traveler recently was given a car with a half full tank. Not to worry, said the agent, just bring it back half full, there's a problem with the gauge. At that point the renter should have asked the agent to note the alleged gauge problem in writing on the paperwork. Sadly, he did not. When the car was returned half full in a different town to a different agent, the renter was charged for 10 gallons of gas, $157. He didn't discover the charge until he was back in the U.S. and received his credit card statement. Since he had nothing in writing, he was unable to successfully dispute the charge.
Lesson: When picking up a rental car anywhere, make sure all anomalies—scratches, less than full fuel tank, etc.— are noted in writing.