Damn. I feel your pain. Same GD thing happened to me a few days ago.
Playing a online MTT, 109 entrants with late registration. I build up a massive chip stack compared to most of the field by playing a solid, patient TAG game. Picking my spots well, stealing blinds and antes, and only showing down very strong hands, not getting involved with too many marginal hands.
We get down to 28 players and I get moved to a table with the chip leader, his 43-46k to my 38k. At this point, I have something like 65BB IIRC, so I should be sitting pretty. Unfortunately, the chip leader is sitting to my left. Long story short, I raise, we get into a raising war and it all goes in on the flop. My set of 8's to his set of jacks. Of course I lost. I donkey away the second biggest chip stack, with a great shot at the final table, all because I got too cocky and didn't take the time to think the hand through. I didn't even CONSIDER, that fact that he might have a bigger set. I was so mad at myself. Played about 4 SnGs since, and have cashed in 3 of them so I guess I learned my lesson.