I accidentally stole a load of shit from my friends when I moved away from the states
At my friend Justin's house we both had all our Pokémon cards spread out all over his room's floor, and when the time came for me to go home I threw all mine together. When I got back it transpired that I had a fuckload of his in there, too, and I never got a chance to return them. Later on, when I returned to say hi to people around six months down the line, I had my copy of Pokémon Stadium at another friend's house while staying the night. When the time came to haul ass back to the hotel, I saw a copy on the carpet and stuffed it in my bag. Got to the airport and opened my bag - Two copies of the game were in there.
I still feel guilty for never returning it.
And, oddly enough, the third stealing incident I can recall was also Pokémon related, except I was the victim this time. I was the first person in my school to have an English language copy of Gold, and had it with me in the school computer room. I left it by my keyboard, left the room for twenty minutes to say hi to my older brother (who'd just came down from the accompanying high school - strangely I am now the same age as he was when he came down to visit me, and I'm living in his old dorm! creepy). Came back in and someone had nicked it; never found it again. Worst part is that this was a prestigious private prep school filled with rich kids with overpaid divorced fathers who'd just buy them the damned game anyway, but noooo, turns out they were never taught stealing was wrong. fuckers
edit: Oh, and not exactly stealing, but in 2004 I'd left a glass of water and my phone next to each other on the table in my dorm. I got back from a run and some total mother fucker had fucking PUT MY FUCKING PHONE IN THE FUCKING WATER. The screen was dead and it didn't do anything apart from incessantly vibrating when I turned it on. fucking housemaster didn't even make a fucking effort to find out who did it. ahhh
Jam it back in, in the dark.