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ID122-Mah Jongg In Error with a Verbal Declaration of Mah Jongg and no Exposures
Question: If a player declares Mah Jongg but has not exposed any tiles, can they change their mind?
Answer: Yes, if a player declares mah jongg but has not exposed any of their tiles and no one has disturbed their hand, they may change their mind and continue playing without penalty.
Source: Mah Jongg Made Easy 2020 pages 16 #2