I was recently playing against a player called artiguy. We played a close match and reached the point where the game had effectively ended and several stones of his were dead and trapped within a living group of mine. It was unclear who had won or lost at this point. When we both passed to enter negotiation he would mark my dead stones and then reject my markings of his dead stones. He would then pass again but I was forced to place unnecessary stones because the passing button was grayed out. Ultimately, I lost by two points after being forced to play 8+ unnecessary stones. While I care very little about the result of the game as the score reflected that I should have won, this is a problem that should be fixed to prevent further exploitation of the existing system.