The cancel button lags behind the client.
When you click cancel, the cancel message still needs to get to the server before it will cancel your move, therefore if there is lag in the system the cancel button will appear to not be working but in actual fact the signal just hasn't reached the server before both players picked a move.
So just be extra sure about your move when you are in a laggy enviroment instead of relying on the cancel button when the entire system is lagging -.-
@kupo: jioo mkn
@kupo: hi mjb
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