Encountered a bunch over the years directly and second hand, but the most recent one that sticks out was from a friend who's a drum teacher at a fancy private school. He's not technically employed by the school, so he bills the parents directly. Went to check his account and found a few more 0s than there should be in his teaching account. Turned out he'd accidentally billed one set of parents 40'000 instead of 400 and they just paid it anyway.