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ID109-Discard-Calling a Discard that a Player Threw Before They Picked
Question: If a player discards a tile prior to picking, can another player call that tile for an exposure.
Answer: No, if a player discards before they pick, their hand should be declared dead and the discarded tile would not be available to be claimed by anyone.
Source: Mah Jongg Made Easy 2024, page 19(g)