Question: What happens when two players call the same tile and the one who was entitled to the call displays an invalid Mah Jongg?

Answer: The player next in line to get the Mah Jongg would be entitled to the tile. If the player was not granted the win, no one would throw the winning tile, putting the player at an extreme disadvantage.

Source: Mah Jongg Made Easy page 20, 13(f)