A classic game, forgotten only because of the accident at the end that allowed Black to salvage the draw.
In a heavily tactical middlegame, Petrosian sacrificed first an exchange, then with two pieces under attack, moved a third one under attack also, all as part of a plan to bring about a "Windmill" combination of repeating checks, similar to the one from Torre-Lasker, 1925.
At the crucial moment, with the combination executed, but with the flag on his clock about to fall, Petrosian inadvertently stumbled into a 3-time repetition that allowed Spassky to escape with his hide. (The 3-time repetition rule was not used in the Soviet Union outside of FIDE events, and Soviet players sometimes forgot to apply it. See also Fischer-Petrosian, 1971 and Castro-Petrosian, 1976 for two other cases where the 3-Move Repetition rule bit Petrosian. See Petrosian-Spassky, 1966, Game 22 for a case where it came to his aid)
Despite the conclusion, this game still has some claim to being the best one of the 1966 World Championship Match. Note by Mikhail Tal, unless stated otherwise.