I'm not a Go player but watched some of the famous Lee Sedol match on youtube while it was happening. One channel had a Korean professional 9-dan (= super grandmaster) commentating. At one point in game 1, AlphaGo made a move that shocked the professional. He spent a while analyzing it, asking whether it could work, etc. The next few moves were routine, so he commented on them briefly and then went back to talking about the surprising move. More moves came and he commented on those. Then perhaps 20 minutes later, during the wait for another move, he went back to the surprising move and said "if that move works, AlphaGo is going to win all 5 games". It was so far-reaching and subtle that if it was not a mistake, it could only be made by a player of superhuman strength.

As we know, Lee Sedol managed to win game 4 by a lucky break, but lost the rest, so the GM's comments were incredibly perceptive. I took a few notes while watching so I may be able to dig them up and check the details, but the surprising play became quite well-known and involved something called a "dog's head" formation, which might be enough for Go aficionados to identify it.