A fortnight ago I wrote about Luděk Pachman's book, Decisive Games In Chess History. One of the tournaments that Pachman looked at was the Capablanca Memorial at Havana in 1965, which Borislav Ivkov would have won had he not disintegrated at the end. Leading by a point with two games to play, he lost both.
This is his penultimate round game against the back-marker, Gilberto García: position after 36.g2-g4. It's hard enough to lose this position for Black, but Ivkov managed not only to lose, but to play the worst move on the board.
There's only one way to allow White a forced mate in four moves. Ivkov found it - can you?
[Thanks to Angus]
[Worst move index]