whats the stupidest thing youve done in chess?

heres one of mine:

Blundered into many mates in 1 online.

Blundered a queen in a club game... TWICE in the same game.

Blundered all different kinds of material in tournament games.  Never a queen for nothing though.

play the ruy lopez chirgon cd varition always a draw

In the game below I am sure I did a lot of stupid things, but I should have had at least a draw but I went for a win.  On my last move...

heres another one:

its completely embarassing because it was the game for first place. I had outplayed my superior opponent and was up Queen to a Rook.

Well i under estimated my opponent and fell for a mate in one.

well that was that.

Okay, so far macer75 wins (and post #1 is in 2nd). 

http://www.chess.com/forum/view/game-analysis/im-gonna-kill-myself

I had mate but cocked it up and got mated myself!

lol... thanks?

As for my own "stupidest thing," I'd have to pick the following.  Nearly 40 years later and it still galls me!  This was the tenth (and final) game of my match against the champion of my high school.  I had been down 4 1/2 - 2 1/2, but then won games 8 and 9 and got to this position in game 10.  I was still only a freshman too, so my nerves were definitely kicking into overdrive:

You must mean #7 (since he has two).  I preferred the other one because it was a rarer idea, although obviously #7 is the tops for turning a win into a loss... 

One of the stupidest things i have ever done was sit down at the wrong board last year at a major tourn. played about 15 moves before realizing it i know it may not be much but it made me feel like a really big idiot

Played chess.

This is an OTB game with normal time control. I was in slight time trouble (5 moves for about 1 minute), but my opponent took his time thinking, so I was able to check and re-check the entire combination in mind. 

After he played 2...Kb8, I followed my plan and immediately grabbed the Queen and placed it "somewhere far away on the a file". Then, without giving the board a second glance, I started to write down the latest moves on my scoresheet.

There were some people around us attracted by the excitement of the time trouble and there was the usual buzz from them in these situations. However, while writing the moves I sensed that a deadly silence fell on the audience and it made me wonder what's all this about. Only then I looked at the board and realized that I put the Queen on the wrong square...

Whenever I am up material i.e. at least a minor piece I always seem to lose concentration and let my opponent right back in the game

The stupidest thing I did happened right after the smartest. I got a clear won game against a player 300 + point above me in Las Vegas. Then I had to seal a move. It was an illegal move causing me to forfeit. He was about 2400, me less than 2100. I'm still pissed. 

Earlier today I spent about 10 minutes carefully calculating the lines of a tactics training problem on another site. The first move was Qh4+, using the support of a white pawn on g3 to check the black king on g4 and drive it into a mating net. There was a black pawn on g5, but it couldn't capture the queen on h4 because it was pinned by a rook. After verifying all the lines after Qh4+, I proceed to play Qh3+, where the queen was totally unprotected, and the king just captures my queen.