I left my SP and several games outside once. Some kids took it. I had to pay another kid $20 bucks to get it back. Turns out he was the one that took it, and he hadn't played it. His MOM was playing it for a bit. (Cue sad music)
Casualties:
My Mario Bros: Superstar Saga save (I was on the last battle!!!). The thing is, the other two save slots were free. But no, the kid's mom DELETED MINE!

Pokemon Blue: I had almost everything in this. Level 100 Blastoise and others, gone.

Pokemon Gold: Not quite as far, three level 50s. Poof. I find two pidgies instead.

Rayman Advance: All gone. I wasn't far anyway.

Megaman Battle Network 2 barely made it.

LoZ:LttP got toally wiped. All done except four swords.