DECLARING MAH JONGG ENDS THE GAME (Article 49)  

Commentary: Once a player declares Mah Jongg, the window of opportunity to claim the last discard closes, and the game ends—provided the hand is valid. No one should throw in their tiles prematurely, as the declared hand might be invalid. The declaring player should fully expose and neatly arrange their tiles according to the card as a courtesy to others. The remaining players then verify the hand. If the hand is valid, the player is declared the winner and receives points. If the hand is invalid due to a false Mah Jongg, the player is disqualified, does not discard, and play resumes with the player to their right.

Question from a player:  What happens when someone declares mah jongg?  
 
Answer from the NMJL: No player should throw in or expose their own hand until the winners hand is verified.

Source:  Mah Jongg Made Easy (2024 page 21 #1)

Other answers from the NMJL:
 
“Do not throw in your hand when an opponent declares mah jongg.  First verify the mah jongg hand.” 
 
“When mah jongg declared, no player should throw their hand in until it has been verified.  If one player does throw in their hand and breaks the wall that player is responsible for the stopping of the game and that player who broke the wall pays the two remaining player the lowest value on the card.” 
 
“The player must pay the two surviving players the lowest value on the card and the game ceases.” 
 
“Mah jongg in error.  No player should throw  their hand in until mah jongg has been verified.” 
 
“Mah jongg in error.  No player should throw  in their hand or expose their hand until the declarer’s hand is verified.” 
 
“Mah jongg in error.  No player should throw their hand in until mah jongg has been verified.  If the declaration of mah jongg is in error, and one player throws in their hand and breaks the wall, that player is responsible for the stopping of the game.  That player who broke the wall pays the two remaining players the lowest value on the card.” 
 
“Player #1 already had her penalty by not being able to continue to play.  therefore, player #2 who declared mah jongg in error would pay the remaining player double the value of her incorrect hand.”  
 
Sources for other answers:  National Mah Jongg League Bulletin Q&A (1967 page 5, 1968 page 5 , 1971 page 5, 1972 page 4, 1973 page 4, 1975, 1982 page 3, 1983 page 4, 1984, 1985, 1987 page 4, 1990 page 5, 1990 page 13, 1991 page 5, 1993 page 12, 1997 page 5,  2013, 2005, 2006, 2019)