This actually happened to me at a pizza place a few months ago. I grabbed one slice, paid with a $10; the register guy hands me back something like $17. Unlike TS, I'm not a broke degenerate, so I told him I was fairly positive I'd just given him a $20 and that he'd over-changed me. He scratched his head for a minute and then stood up and went to the back. I presumed he was checking the camera footage, and proceeded to eat my slice. Ten minutes later, he'd never returned, so I left.
$16 short, dumbass.
Edit: $40. I'm the dumbass.
You knew the idiot fucked up and you giggled as you walked out of there with extra money in your pocket. You're a thief.
Lo that gif with your avatar.
Unless this happened somewhere on Pines Road in Shreveport, LA, I'm sorry to inform you that your search for dumbest service industry worker has been fruitless, so far.
Edit: what a dumb ass though. I hope you both said thank you.