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: In the event a player calls a tile, whether for exposure or for Mah Jongg, at the same time a player picks a tile from the wall, the player who picked from the wall has to put the tile back exactly from where it came.  This ruling is not found in Mah Jongg Made Easy. 

Source: National Mah Jongg League Bulletin Q&A (2020)