I had a similar experience... twice:

In Detroit last fall, their computers were down and they gave me a blank contract with the only things listed being my initials and "Ford Explorer". I got pulled over in IN and the officer was P155ED that I didn't have any other proof that the vehicle belonged to me. Anyway, I took the car back 3 days later and the computers were still down! Our fiscal year ended in October, a few weeks after the rental return so I couldn't let this wait for the following year. I called the CHM desk and they couldn't find any record of it. I could have just hung up, but they got the date/time out and in and told me they'd have processed shortly. They called back and told me they put the charge through and gave me a $50 credit "for being patient." I'm guessing if I never called, they never would have charged me and I wonder how many people got free cars that week.

In EWR this spring, our flight was cancelled and I had to overnight. I made a last-second reservation as I walked through the terminal and got to the garage to find an STS in my spot. I sweet-talked the agent into an M35x. When we returned it, the scanner "couldn't locate it" at the check-in garage. We were in a hurry and I asked the guy to just give us the valet drop, and check it in later. A week later, I never got the bill and wasn't too worried about it (see above). However, I went to our local station in TX for another rental and she said I still had the EWR car out and didn't want to rent to me unless I called to close it out. I called the CHM desk and said I returned the car within 24 hours of picking it up, just so I could get my new rental car off the lot. It was another week or so 'til my email receipt came through.