Sorry for the typos. I was in dictation mode and did't edit before posting.
You could beat Magnus, Bobby, Tal et al

One single move against Carlsen wouldn't make a difference.
he would still outplay you in tactics and strategic understanding.
maybe if you are 2300 rated a single move would matter.
in the simul http://www.chess.com/news/swift-magnus-carlsen-beats-10-but-loses-1-in-chesscom-simul-5982
a single move made the difference.
I recently was comparing my chess game to my golf game. At golf, I managed to get down to a four handicap. My best score was a 69, and I had two holes in one. I don't play as much as I used to. I do now play a lot of chess. It may sound strange, but it often compare the two sports. Each takes practice, preparation, and a lot of play. Even though my four handicap was never near being a pro, it was good enough to experience pro like moments. The two hole-in-ones, for instance, were pure. By around 69 was bogey free, and I sank quite a few nice puts.
So I am going to take that experience and extrapolate to chess. In chess, my rating has been as high as the mid 1500s. Like my four handicap, that has given me the chance to play some games I've really enjoyed. So I'm going to say that anyone with a 1200 or better rating could actually have a game where they beat the likes of Magnus Carlsen, Bobby Fischer, order me a Mikhail Tal. In this game they make exactly the right move at the right time. Perhaps they did not reason it out like a grandmaster would, and it was just luck that they made that move, but I still think it's possible. Not to be sure that the odds of this happening are very, very small. But I do believe they exist. So, take heart. You could beat Bobby Fischer.