Question: What occurs if a player mistakenly declares Mah Jongg but another player actually needs the discarded tile to complete their own winning hand?

Answer: If a player calls Mah Jongg in error and another player requires the discarded tile to achieve Mah Jongg, the player in need of the tile has the right to claim it. 

However, if after a Mah Jongg in error, a player needs the discard for an exposure rather than for Mah Jongg , the tile cannot be claimed. The tile remains with the player who made the erroneous Mah Jongg declaration. See ID #199.

Source:  Mah Jongg Made Easy 2024, page 31, #4