Polugaevsky-Tal, Soviet Championship, Moscow 1969
In February 1969 Spassky and Polugaevsky had a joint training camp to prepare opening ideas for upcoming important events. This turned out to be very prolific work. The two Grandmasters analysed deeply for White one particular variation in the popular Semi-Tarrasch Variation, finding a poisonous arrangement of white pieces, which was very dangerous for Black.
Spassky was the first one to profit from their analysis, by winning the fifth game of the World Championship match against Tigran Petrosian. Later in the Soviet Championship, in the encounter Polugaevsky-Tal, Black improved on Petrosian's play, only to allow Polugaevsky to demonstrate the depth of the main analysis from the mentioned training camp in February. In a typical pawn structure, White sacrificed a central pawn for strategic goals. Later it became standard in such positions, but in 1969 that particular sac was new. Polugaevsky got a long-term positional compensation for the pawn, and at the same time set a diabolic tactical trap. Tal ran directly into it and allowed his opponent to show the tactical potential of White's position.
This game had an important theoretical impact for many decades, until new possibilities for Black were found and the Semi-Tarrasch became a popular opening choice on the high level again.