My opponent blundered a Queen, undisputed.
I failed to see a 1 move checkmate that I had with a rook and Queen pursuing his king-side castle, I had him on the ropes. I had people watching, everyone knew what to do but me, and I screwed up xD
He got his Queen back and checkmated me a turn later without me knowing, and the cream on the cake was that I realised the move I had to make the very second I lost
I know how you feel man xD
You know those won games you manage to screw up? Well, that's one of them. Indeed, that's a masterpiece within such category. It requires a lot of talent to make so many blunders in an endgame after accumulating a great material advantage during the middle-game.
I think part of the problem was overconfidence. Hubris indeed. I had won five games in a row, my rating reached 1606, the best I ever had so far, and I have beaten that same opponent easily in a previous game. By the move 15 I had two pawns and a bishop of advantage (indeed, I had the pair of bishops and he had only a knight, what means my advantage was of seven pawns!), then I thought the game was already over... Couldn't be more wrong, as in the move 47 I resigned owning one rook against two of my opponent...
Here's the tragedy: