It has happened to me once like 5 years ago. I picked up my car and went to my destination. Everything seemed to go all right. I get a call the next morning asking me if I was going to pick up my rental. I tell them I already have my rental and read the rental # off of the key fob. I end up returning the car at a hotel like I scheduled. Never got charged for it as far as I knew.
Yes, I have also had this occur ONCE. Was like a 6 day work rental, was like a 480-500 dollar estimated rental (those are usually pretty close) and realized i was never charged. Called and asked for reciept, before I realized it, never was charged or had to pay it. Was rented/returned to same airport location.
Too bad it was not on a fun trip!