Kasparov and many others thought the move was too sophisticated for a computer, suggesting there had been some sort of human intervention during the game. “It was an incredibly refined move, of defending while ahead to cut out any hint of countermoves,” grandmaster Yasser Seirawan told Wired in 2001, “and it sent Garry into a tizzy.”

Fifteen years later, one of Big Blue’s designers says the move was the result of a bug in Deep Blue’s software.

http://www.wired.com/2012/09/deep-blue-computer-bug/