Experienced this with Avis on the one occasion I rented from them. Eventually the scratch that they said I had put on the car was found to be already there when I collected it (early morning, in the dark, from Manchester T3). Hertz tend not to do it, but then I have rented from them about 70 times recently and I know the staff at T3 so well that they overlook the odd car park ding or hedge scratch that the cars are returned with.