I fueled up an N3N I had been flying at the local FBO after hours once where you just run your credit card through and it gets charged. But, the machine wasn't working right, and I got the $75 worth of 100LL for free.
The FBO owner was a bit of a cheap person and not real popular, so maybe some thought it was great getting a little free gas. I went back a couple days later when I had the chance, told him what I owed him and paid him. He was happy, seems I was one of the few that did that.
dago