Back when I played Ultima Online (I was only about 11 years old at the time) I discovered a very interesting 'feature'.
You were not allowed to kill other player characters in towns, and if you did attack someone all that person would have to do is say "guard" and a guard would pop up out of nowhere and instantly kill you with only one hit. I found a way to use this to my advantage. There was an ability that would make a monster attack it's monster friends, or if the ability was unsuccessful, the monster would instantly target you instead. Well this ability could also be used on other players. So what I did is I would cast this ability on another person and it would fail because my skill level was too low, and then all I had to do was stand next to that player and s/he would automatically start to attack me. At that point I would just yell for the guards to come and it would kill them instantly, leaving only me and their body that I could loot and steal all the things they had with them at the time. Eventually after using this tactic I got caught by an admin, banned and the players had their things returned to them. I totally agree that I got what I deserved, but it was still fun as hell to watch people's complete and utter confusion as they attack me for no reason and then their items just disappear into thin air.
I know this was pretty much just cheating by exploiting a bug I had discovered, but it was definitely a new and unintended way of playing the game.
The first rule of intelligent tinkering is to save all the parts. -- Paul Erlich