Mine has to be Red Dead Redemption's "Dastardly" achievement: Place a hogtied woman on the train tracks, and witness her death by train.
I didn't even know it was an achievement. I just wanted to see what would happen. And I think that's pretty common among people who earned that achievement. It's like Rockstar knew everyone would try it.
Game Conquerors Top 100 Games of All Time http://www.youtube.com/IveConqueredThatGame