Petrosian - Bronstein
By all accounts, Petrosian had a very strong position against Bronstein who had just played 35... Nf5 from d4. Inexplicably, Petrosian played the howling blunder 36. Ng5??. Brontein must have been incredulous but played 36...Nd6. Petrosian resigned. It just shows you that everyone is prone to a catatrophic lapse of attention.