When is a player committed to taking a tile from the wall?
If a player moves the tile or lifts it, they are committed to taking that tile from the wall. However, if the tiles has been touched, but not moved in any way, the player can change their mind. Until that tile is properly racked, other players still have the opportunity to call the last discarded tile. If someone does indeed call the last discarded tile, and the wall tile has not yet been racked, the player would need to put it back in the wall in exactly the same place, regardless of whether the tile has been looked at or not.