Question: Someone discarded a tile and misnamed it. However, they had the correct tile in their hand. What happens in this situation?

Answer: A player cannot replace a discard with the correct tile from their hand. Instead, it should be brought to the player’s attention and then the tile should be correctly named. Any player can rename it. Once a tile is named or hits the table, it is down. 

Sources:  Mah Jongg Made Easy 2024, page 16, #3 and page 17, #4