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ID98-Simultaneous Call for a Discard and the Next Player Picked and Racked
Question: During my game today, a player called a discarded tile at the same time another player picked their tile from the wall and racked it. What happens when this occurs?
Answer: The player who picked the tile from the wall has to put the tile back. This ruling is not found in Mah Jongg Made Easy.
Source: National Mah Jongg League Bulletin Q&A (2020)