Winning a game in 1 move

I once won a game by checkmate after playing only 1 move. "Impossible" you say. In the normal course of affairs you would be right, but this wasn't a normal game. It was part of the Belconnen Chess Club's 'Gambit Allegro' which was a fun tournament we held once a year.
The way it worked was that there would be an envelope containing slips of paper with opening variations written on them. As the tournament name suggests, these openings were all gambits, and before each game one player would reach into the envelope and pull out an opening. The game would then start with the specified moves on the paper.
In one of my games the opening chosen was the From Gambit. So following the script we played 1.f4 e5 2.fxe d6 3.exd Bxd6 4.Nf3 g5 (D) This was where the moves ended, and as I was Black I started my opponents clock and awaited his first move. After a moments thought he played 5.h3?? which allowd me to play, as my first move of the game, 5. ... Bg3# In a sense, my quickest win ever.