Congratz for the win!
In simultanious games it can happen. In the recet simul. games gueiness record GM Alik Gershon lost to a mate in 1 in favorable position (piece up I think)! of course he was playing around 520 games and he was still playing close to 50 when he lost, but it's still a simple mate in 1! (I got a draw)
You can also often find blunders in GM games.. I remeber seeing a game of Kasparov losing a piece to a simple fork in the early part of the game..
In conclusion - YES, GMs are humans aswell...
The GM Jonny Hector was visiting my local chessclub yesterday giving lecture and ended the evening with a simultan chess vs. 12 of our members.
I ended with a surprising win and will never forget it. Yes I was lucky, but still a win vs. a GM!