Example of a Karpov vs. Kasparov game with a controversial move Full disclosure: I'm not a regular chess player. This question is because I'm writing a book about alternate timelines, and I want to use a game in a real life famous chess match as a point of divergence. I'd like an occasion in which a sizeable portion of pundits and specialists thought a different move should have been made, and the player who made that move lost the game, or squandered an allegedly easy win for a draw.
It'd be great if it was in the 1984/85 Karpov/Kasparov championship match, but any headlines-making match at any point in history would do. It could be Kasparov vs. Deep Blue, or Karpov vs. Korchnoi, for example.
If I use your suggestion, I promise I'll properly credit you. And the players involved. Even if it's a computer.
If you don't mind going a bit earlier, then it has to be 1972 Spassky–Fischer (mother of all World Championships) Game 1.
Fischer played 29...Bxh2? a move that few players would consider in light of the obvious 30.g3, trapping the bishop.
EDIT Later analysis (Speelman 1981) showed that the game is not lost (and can be drawn with extremely precise play), though the move was sub-optimal and the line becomes really sharp after 30. g3. The real blunder happened on move 37 with 37...Ke4 instead the optimal move again was just 37...a6. However, going by principles and elementary tactics 29...Bxh2? was an unnecessary move and chances are Fischer overlooked the fact that the trapped Bishop couldn't be saved.