That's right. 1996, Anand-Kasparov. Here's the game if u like:
http://www.chessgames.com/perl/chessgame?gid=1018678
Before the blunder, after Anand's 33.Re1 , Kasparov (black) was with a decisive advantage of around -2.50
After 33. ... Qxe3??, Anand gets the advantage, so that was a more than 3.0 points (engine evaluation) mistake.
It's nice to see how even the best player ever can collapse psicologically. Note that after the mistake white is better, but only slightly. The real mistakes came few moves after, to finally lose the game. They had even equal position at some point.