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: If a player calls a tile whether for exposure or for Mah Jongg, 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)