DeepMind’s program AlphaGo lost in the fourth of five matches to human world champion Lee Sedol (9-dan pro), the top Go player of the past decade, in a $1M 5-game challenge match in Seoul.
AlphaGo had won the first three matches.
In October 2015, AlphaGo became the first computer program ever to beat a professional Go player by winning 5-0 against the reigning 3-times European Champion Fan Hui (2-dan pro). That work was featured in a front cover article in the science journal Nature in January 2016.
AlphaGo made an error on move 79 and did not realize it until move 87.
Perhaps Lee Sedol is discovering a playing style that will cause AlphaGo significant problems or at least challenges.