MAH JONGG IN ERROR WITH A PARTIAL EXPOSURE (Article 55) 

Commentary: When a player declares mah jongg after picking a tile from the wall and exposes tiles from their rack only to realize they are not ready to win, the player is disqualified. Invalid exposures are returned to the sloped part of the rack. Prior valid exposures stay on the rack, and jokers, if any, are eligible for exchange.

If the player declares mah jongg on a discard and either places the discard on the flat part of their rack or exposes tiles only to realize they are not ready to win, they can retract the declaration. They are committed to the exposure, so they cannot put the tiles back in their rack—they have to make it work. If they cannot follow through, they have mah jongg in error and must stop playing.

Question from a player:  I picked a tile and declared mah jongg.  I placed the tile on my rack along with two tiles from my rack.  I then realized I did not have mah jongg.  Can I change my mind regarding mah jongg or is my hand dead? 
 
Answer from the NMJL: If a player declares mah jongg on a discard and places the discard on top of their rack or exposes tiles from their rack, and then finds they do not actually have Mah Jongg, they may change their declaration of mah jongg to a call for exposure. The game continues without penalty.

Source:  Mah Jongg Made Easy (2024 page 21 #3; page 22 4[a], [b])

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Other answers from the NMJL:

“If a player calls Mah Jongg in error and exposes part or all of the hand and all other hands are intact, the game continues but the declarer’s hand is “dead,” and all tiles are returned to the rack.  A Mah Jongg hand “intact” means that a player has not exposed any tiles from their hand, other than the tiles that had been called for an exposure before the player had declared Mah Jongg in error.  The same penalty applies for calling a discard and making an incorrect exposure.  “Dead” hand discontinues play, does not pick and discard and pays winner full value of hand.”

“Your hand is not dead if you can use the discard for a valid exposure and continue playing.  The hand would only be dead if you proceeded to expose tiles that were not part of a valid mah jongg.  When the card refers to mah jongg in error, it means the player has displayed an incorrect mah jongg but has realized the mistake and states the fact that they do not have mah jongg.”  Superseded by the rules in Mah Jongg Made Easy (2020 page 16 #3 [a], page 17 #4 [a]).

Source for other answers:  National Mah Jongg League Bulletin Q&A 2020