In last year's SIO, former Australian Junior Champion (1982) Paul Broekhuyse resigned his position after a knight fork. Except that his opponent's move was actually an illegal one.
In today's first round of Sydney International Open 2010, it happened again, this time to no less a seasoned player than grandmaster Daryl Johansen!
Ansell, A - Johansen, D., 1-0
It looks like that knight made the supposed-to-be-impossible f4-h2 maneouvre. And now: 32... Rb4 33. Nf3 One... 33...Nh5 34. Ng5 Two... 34...Nf6 35. Ne6 BANG! The GM is kaput! 1-0
Doubtless both players were in severe time trouble and simply overlooked the mishap. Such is chess.
Coverage of the SIO tournament is here.