How often do you check what's billed to your credit card?
I had a Hertz rental at LAX, and when I returned it the receipt said $40.00. A couple of days later it was the end of my billing cycle for my credit card and on the other charges was a charge from Hertz for $47.00. I went to Hertz web site to look up my receipt and they charged me $7.00 for fuel. I did re-fuel the car. I call the customer service number and they put a credit for the fuel charges on my credit card.
Does something like this happen often? If it wasn't for the end of the billing cycle, I might not have caught it. Do you check your statement to make sure that the charges are correct? I've never had that happen before, but it seems that I do have to fight at LAX over whether I re-fueled or not. I always show my receipt for the gas I put in.