Professionals are understandably hesitant to call AlphaGo's plays mistakes after the first few games, because AlphaGo had played a few moves that seemed like obvious mistakes (the eyestealing tesuji under the avalanche and the fifth line shoulder hit) but later proved to be very strong moves. Remember that when go is your career, your reputation is based partly on your ability to analyze games accurately, so calling a move a mistake and finding out later that it was strong would be a big embarrassment.