It's a tad annoying that it seems like there are so many A players (I'm an A player) and B players that have beaten so many strong players that I never have beaten -- the best I have done was a draw against a 2100, and a draw against a 2200 who wanted to win clear first so only played 8 opening moves and offered one to me :) I have however beaten lots of 2000s.
I'm kidding of course . Some people are just more consistent than others -- some players are the kind of people who will beat IMs and then lose to a 1600 the next round :)
But there have been plenty of upsets involving more than 400 points before -- I remember in one tournament a year or two ago (it was posted here on chess.com, either as an article or blog), GM Arun Prasad, who had a FIDE rating above 2500 at that time, lost to a girl only rated in the 1900s. She was a young girl, though, so it's not impossible that her strength was higher than her rating suggested. Still, 600 points...