DrMAL: Not sure why this is not listed as one of the most memorable games of the match, it certainly was to anyone following it. It was likely the decisive one.
The opportunity missed here, quite possibly the move that decided the match, came from the players themselves (http://www.youtube.com/watch?v=RQmZ...)
It was 28.f4 instead of 28.Rc2 described rudely by Kasparov as a "typical Karpovian move." Karpov politely conceded that he missed 28.f4 and it was "probably winning."
For myself, it seemed clear that white had a solid advantage after 28.f4 but it was unclear if white was "probably winning." Rybka 4.1 evaluates these two moves as follows (lines truncated).
After 28.f4: [+1.15] d=21 28...Bg4 29.Bb5 Rc7 (0:35:01) 226612kN
After 28.Rc2: [+0.25] d=21 28...a5 29.Ra4 g5(0:14:52) 100417kN
I still am not convinced this move was so decisive but what matters is what the players thought at the time, they did not have the luxury of a computer program of anywhere near the caliber of today's engines.
In the game that followed, Karpov made two early mistakes and lost quickly, indicating his psychological change from this mistake. On game 17 he had re-composed himself and went on to crush Kasparov but this only tied up the match.