my brain got dumb during the game. Lost the game as a result.
what's the dumbest way you've lost a game?

There is a psychological phenomenon, in chess we call it "from winning to losing".
It's a kind of subconscious self-sabotage where you are almost purposely trying to lose after acquiring a winning position. You'll know how devastating it is if you have experienced it before.

Probably the time in the final round of an over the board tournament that I’d travelled 4000 miles to, instead of choosing any of about 5 easy wins, I picked the only move that allowed my opponent to mate me in 1 move. This was after a 15 minute think as well - so it was quite stupid.
Make a blunder in an overwhelming position, and then immediately resign, without realizing that I was still winning. Full Tilt.

I had a drawish position in an online game, but I double clicked the white flag instead of the white glove on a poorly labeled game screen. I heard that the Women's World Champion made the same mistake in an important online game.

Ok, I'm glad it's not just me!! And, those over-the-board blunders are the worst.... another one I seem to fall for is the silly old back rank mate, especially when I'm about to win... like another said, the one move I choose is the one that allows it... I think sometimes the desperation of being in a losing position opens up your eyes to last resort tactical moves, and conversely the positional winner gets too comfortable and misses it. I've been on both ends of this, in blitz anyway. Lesson: never give up on the position too early, and always, always watch for back rank mate threats!
When I am +5 in front AND still have 6 mins left while opponent has 2 mins yet i still lose via check mate

oh, one more...my brother got paired against me in an open tournament (we were kids) so he thought it would be funny to copy (mirror) all my opening moves...until he lost his rook...

That never happened.
For me: MOUSE SLIP!!! (or finger slip) I guess this is basically a vent because it's happened to me about 5 or 6 times now. My fault, but still.... a disadvantage of computer chess, I suppose! And general clumsiness