This has happened to me many times in the past few years. I am usually rushing through at 5am and fail to check the receipt each time. In most cases, they have refunded me when I caught it. The friendliest Hertz employee at PHX, "Bud" (now retired) informed me that if the car has less than 100 miles during your rental period, Hertz' default is to charge for fuel. They must take a couple additional steps to override the default and that's why this happens so often.