It seems that AlphaGo suffers from the horizon effect, meaning that when it is ahead, it becomes blind for wining sequence that starts with an unusual move. It is great that Lee Sedol has won this game, because it shows that humans still have a better understanding of the game. Relying on a mental vocabular for reasoning about the game, has some weaknesses, it might still prove superior when reasoning about very complex situations like the one encountered in this game. It seems that Lee Sedol took most time to think about this move. Later on in the game, AlphaGo, played some bad moves, probably because its winning percentage had dropped.
Each new user of a new system uncovers a new class of bugs. -- Kernighan