In fact, pretty quickly within a few thousand games, weak players were able to find certain tricks that they could basically fool the AI and figured out how to exploit it and beat it. Also, in Fan Hui versus AlphaGo, so they famously beat Fan Hui 5-0. But then after they published the Nature paper, they invited him to play several more matches against it to see if he could find out any weaknesses in the AI. In fact, he was able to find weaknesses where he was able to consistently beat the AI and they had to patch this before they released on the Nature.