A friend of mine and I were playing Garry's Mod over LAN and we had a few mods installed where we could build more advanced things like Stargate ships and gates. He was hard at work building some ships and I made an entire arsenal of bombs and missiles and I carpet bombed everything into next week. Server crashed a few times too.
Back when I was a kid, there was someone at our school who was stealing kid's Runescape accounts by finding out answers to their recovery questions. He would try to scam people into giving their answers by promising a reward in terms of money and items. I'm not sure what was going through my head but I decided to give him my friend's details instead to find out what would happen and lo and behold his account was stolen! Fortunately most of his stuff was safe in his bank but it was a pretty dick thing to do regardless.
Not a video game but I used to play Yu-gi-oh over the phone sometimes when I couldn't hang out with my friend. I took advantage of the fact that we couldn't see each other so I sometimes went through the deck to draw the cards I needed so I could win. Cheating? No! Heart of the cards!







