I am, was, is an idiot.
My freshman year of college, I had brought my gamecube games to college with me, but forgot my memory card with all my save data. I had an old gray one with 59 blocks and no save data, so we just started a new save in order to play some multiplayer. Smash Bros. Melee was a gaming event and we played it for the first few weeks of the term.
I still didn't know my housemates too well. One of them was an incredibly good Melee player, beating my butt up and down the floor everytime we played. My card was normally plugged into the machine. So when I fired up a new game (Baten Kaitos Origins) I had to delete a few save games to make up room for it. I chose smash, thinking my friends wouldn't mind now that I had my real card.
Without my knowing it, the guy who rocked at Smash had gone home and gotten his memory card, with eveything unlocked on it. I deleted his smash save, thinking it was mine.
Needless to say, when he confronted me about it, I was quaking with fear. He was angry, but surprisingly forgiving. While he could never get his hours back or his entire collection of trophies (yeah, he had all of them), he did beat and unlock everything within two days and then for good measure, unlocked a few things on my melee save file on my actual memory card.
And surprisingly, he and I became quite good friends.