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

Answer: As long as the player has not discarded to complete their turn, the correction can be made. Once the player has discarded it is too late and thus, 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 an incorrect exposure is noticed, the player’s hand should be declared dead. The player who received the Joker during the exchange is not penalized.

Source: Mah Jongg Made Easy 2024 Edition, pg. 24