If a player completes an exposure and then brings other tiles to the flat part of his rack thinking he might have mah jongg but declares, "I can't do that," he can return those additional tiles back to his rack as long as he has not verbalized mah jongg.
It is preferred that you place a called tile on the flat part of your rack and not in your rack among your tiles. This will prevent picking up the wrong tile for expous
When a player calls a tile and places it on the flat part of their rack or exposes tiles from their rack, they are committed to taking that discard. They may add or take away from the exposure before discarding.
If a player inadvertently exposes the wrong tiles or exposes too many jokers, they can add to or take away from the current exposure as long as they have not discarded. After a player has discarded, exposures on their rack cannot be changed.