Question: What happens if a player has the wrong number of tiles prior to East discarding their first tile?

Answer: If a player discovers they have the wrong number of tiles after the deal but before the Charleston, all tiles are thrown in, and the game begins again. There is an exception to this rule – when the player with only 12 tiles is the player to the left of East, that player can take their thirteenth tile as long as East has not discarded to begin the game. This is the reason why players’ should leave the tiles stacked in groups of 4 as they pick them and should count their tiles prior to racking them. 

Source:  Mah Jongg Made Easy 2024 page 14, OTHER TIPS AND RULES FOR THE CHARLESTON, bullet #1