Yes, this happens quite regularly with Hertz when an employee does not mark the fuel level of your vehicle. I suspect they're expecting the busy business traveller not to notice the additional charge. This has happened to me a few times, and in each case, Hertz has refunded my fuel plus a few extra bucks for my trouble, then followed up with a gift certificate (hush money) after I write with a letter alleging the practice is deliberate.

My personal policy to avoid this is if the rapid return folks are non-existant, is to ask a Hertz employee to note the mileage and fuel, and I take his/her name. Noting their name will prevent you from getting charged.