Been there, done that, went back and paid...
I have also had clerks put someone else's gas on my tab a few times... Try explaining that at $1.75 you can not, no way, no how, put $50.00 worht of gas in the tank of a Jeep Wrangler! ($26.00 if it was bone dry!)
Honesty, while sometimes a hard pill to swallow, is the best policy.