Question: What happens if a player exchanges a Joker and accidentally gives the player with the Joker an incorrect tile and it not noticed until after the exchange is completed?

Answer: It is incumbent upon the person receiving the natural tile to verify that they are getting the correct tile. If the player does not verify the tile and it is noticed after the exchange is complete, any player can called that player’s hand dead for having an erroneous exposure. The player who received the Joker during the exchange is not penalized.

Source: Letter from the National Mah Jongg League dated 6/7/2022