Window of Opportunity for a Called Tile (Article ID 107)
Abstract: It is too late for another player to call the previously discarded tile once the next player in turn has either picked and racked their tile or has discarded a tile, whether for exposure or for mah jongg. This is why it is so important for a player to pick and rack their tile rather than hold it in their hand.
Question from a player: When is it too late to claim a discard? If a player has not racked their picked tile, can the previous discard be called?
Answer from the NMJL: “A player shall not be permitted to call a tile for an exposure or Mah Jongg after the player to the right of the discarder has drawn a tile from the wall and racked or discarded. A racked tile is defined as a tile that has been placed on the sloped part of the rack along with the player’s other tiles.”
Sources: Mah Jongg Made Easy 2020 page 19 #11
Other answers from the NMJL:“Once a tile is covered by the next discard it cannot be recalled for either exposure or mah jongg.”
“Once a discarded tile is covered by the next discard, it cannot be called for either mah jongg or for exposure.”
“Once a tile is covered by another discard, it is too late to call the previous tile. There are no exceptions to a joker following a discard.”
“Once a discarded tile is covered by another discard (even if the discard is a joker), it is too late to call the previous 3 Crack.”
“Until a player has either racked or discarded a picked tile, the previous thrown tile is eligible to be called. The drawn tile should be put back for the next player to pick.”
“A tile may not be claimed for exposure or mah jongg after player next in turn has picked and racked or discarded a tile.”
Sources for other answers: National Mah Jongg League Bulletin Q&A (1976 page 3, 1977 page 4, 1978 page 4, 1979 page 3, 1981 page 5, 1998 page 5, 1990 page 5, 1995 page 5, 2000 page 6, 2002, 2003, 2007, 2008, 2010, 2011, 2014,) https://www.nationalmahjonggleague.org