Google DeepMind's AlphaGo system finally showed its wicknesses in handling "unexpected" moves and on Sunday it lost a Go game round from the South Korean champion Lee Se-dol . The loss stymies the AI system's prospects of equaling a 5-0 win against an European Go champion in October.

A move by Lee, described by commentators as "brilliant," was followed immediately after by a mistake by the computer program, leading Lee to say that AlphaGo, though a formidable opponent, has weaknesses or bugs that degrades its performance when faced with unexpected moves. It also did not perform as well when it has to play with black pieces and has to start the board game.

The fifth and final game in the contest is being played Tuesday.

Lee pressured AlphaGo into a mistake that it couldn’t recover from, DeepMind CEO Demis Hassabis said in a Twitter message. The AI company in the U.K. was acquired by Google in 2014.

Hassabis said his team would work out on ironing out the weaknesses identified by Lee, which was one of the main purposes of testing AlphaGo against the Go champion and other opponents.

In the game, players take turns to place black or white "stones," on the 19-by-19 line grid, to aim to capture the opponent's stones by surrounding them and encircling more empty space as territory.