One of the more famous upsets in chess history. In the final round of the 1958 Interzonal, Bronstein, who was undefeated in 58 previous Interzonal games, found himself getting beaten by the 6-13 Rudolfo Cardoso, and knocked out of the Candidates Tournament.
Had Bronstein won this game, Fischer might not have made it to his first Candidates Tournament. Bronstein would have been in, and Fischer would have had a playoff with Fridik Olafsson (who had beaten him in the Interzonal, and who had better tiebreak points) for the final spot.
After this reverse, Bronstein, who had played in the 1951 world Championship Match, and in each of the first three candidates, never made it back to the Candidates Level again. The closest he came was when he apparently qualified from the 1964 Interzonal, but was excluded by the rule preventing more than 3 players from the same country from qualifying.