If a player clicks on an already filled cell that would send the next player to an already-won board if it wasn't filled, no mark is placed but the turn is considered played and valid. The next player can then choose anywhere to play.
Still happens on a different scenario, when clicking on an empty cell from an already-won board. See attached history (replay using elm debugger).
history-23.txt
If a player clicks on an already filled cell that would send the next player to an already-won board if it wasn't filled, no mark is placed but the turn is considered played and valid. The next player can then choose anywhere to play.