Question: What is the rule for changing an exposure?

Answer: If a player inadvertently exposes the wrong number of tiles, exposes too many jokers, or accidentally takes the wrong tile from the slanted part of their rack they can add to or take away from the current exposure as long as they have not discarded.

These are scenarios scenarios that apply to amending an exposure:

1. If you mistakenly place an incorrect tile from your hand into an exposure, you are allowed to return it to the slanted part of your rack and replace it with the correct tile. For example, if you called a 2 Bam and placed it on the flat part of your rack, then took two 2 Bams and a 2 Dot from your hand to form an exposure, but realized you accidentally took a 2 Dot, you can return the rack and replace it with the correct tile. According to a recent ruling from the National Mah Jongg League (November 2024), this action is permitted. This ruling supersedes a prior decision from February 2024, which stated that such a mistake would result in an erroneous exposure. The community is hoping that this will be clarified in the 2025 Bulletin coming out next month.
2. If you mistakenly put up the wrong number of tiles in an exposure, e.g. you put up a Kong of 1 Dots but, it should have been a Pung, it is permissible to put the tile back in your rack.
3. You can never use a discarded tile to add to previous exposures. A called discard must be able to make a new exposure. Source: Mah Jongg Made Easy 2024, page 15,  para.2
4. If you call a discard and pick up the wrong tile and put it on top of your rack, your hand is dead. Source: Mah Jongg Made Easy, 2024, page 19, #15(c).