FOURTH PLAYER FORGOT TO PICK THEIR 13TH TILE (Article 63)  

Commentary: If it is noticed that the fourth player (i.e., the player to the left of East, North) is short a tile and East has not yet discarded to start the game, North can pick their 13th tile and the game can continue. However, if it is noticed after East discards, another player should declare their hand dead. 

Question from a player:  I am the fourth player sitting to the left of East and I forgot to take my 13th tile.  Is my hand dead? 
 
Answer from the NMJL:  If it is discovered, during the Charleston, that any player has an incorrect number of tiles, the hands are thrown in and replayed. There is one exception to this rule. If the player to the left of East is the player missing a tile, that player may take the next tile from the wall, since it was rightfully theirs. This may only be done before East discards.

Source:  Mah Jongg Made Easy (2024 page 14 bullet 1)

Other answer from the NMJL:

“If the player to the left of East has only twelve tiles after picking of tiles from the wall or after the Charleston, only this player may rightfully pick the thirteenth tile before the Charleston or before East has discarded the first tile. ”  
 
Source for the other answer:  National Mah Jongg League Bulletin Q&A (1987)