The Losing Move. Usually used in the context of playing a board game of Go. But it can be used in just about any context you can think of, which allows you to say that something is hopeless, despairing, unrealizable.
He thought that the move was good, it was actually the move playing which the eventual loser still had his chances, but chose a clearly inferior play which was Baizhao. The winner next made no mistakes so that even if the loser didn't make any either, he still could not recover the loss.