My younger brother used to compulsively delete all of our game files. Invariably, if we let him play anything, it would end up deleted the next day. Almost everything I played during the SNES or N64 era was affected at one point or another, and no amount of scolding or yelling stopped him until he finally got a clue on his own.
This is tangentially related, but on another instance, he woke up in the middle of the night and apparently didn't feel like walking to the bathroom, so he pissed right onto our Nintendo 64, with Diddy Kong Racing inserted I think it caused the game to temporarily glitch up (don't ask why I was still playing it after that incident, it was sufficiently cleaned up) because when we started the game later it had unlocked a bunch of characters and tracks that we'd never had before. They went away after we shut down the game, but it was probably the only good that could ever come from taking a leak onto a gaming system
I also had a few carts (DKC2, for example) that would automatically delete themselves after beating them. I've run through the game about 5 times and every time I'll find the file slot empty when I return a week later.