Question:  What happens if a player misnames their discard?

Answer: If a player misnames a discarded tile, it should be correctly named by any player. If a tile is called on a misnamed tile that was not correctly named, the following penalties would apply:

1. Invalid Exposure: If a misnamed tile is called and an invalid exposure is made, that player’s hand would no longer be viable. No penalty is imposed on the player who initially misnamed the tile. To prevent your hand from going dead, it is important to look as well as listen when playing Mah Jongg.
2. Misnamed Tile for Mah Jongg: If a misnamed tile is called for Mah Jongg, the game ends and the player who misnamed the tile is required to pay the winner four times the value of the hand.  Other players do not contribute to this payout.

Source: Mah Jongg Made Easy 2024, page 16, #3