Super Smash Bros Melee.
It was paused whilst someone had a hammer. One of my friends remarked "Hey, that's pretty good, I'll try to learn that--" turns, picks up guitar, stares at it for all of about two seconds -- and plays it perfectly first time.
It's not that difficult. Two measures, three notes in a major triad, and a fairly simple rhythm.
My SSBM moment is not that excellent, but something I'm still proud of. There was a tournament at my school, matches were best two out of three rounds on stock. I never lost a single round until after my third match, where I was thrashed handily by a Sheik player that went on to win the entire thing without losing once. From what I saw, I was the only player to get that far that ever used more than one character (Sheik, Marth, and Samus for me).