Furiten

Furiten is a Japanese-only rule that blocks you from declaring ron if any of your winning tiles has already been in your discard pile. The intent: you should have called ron the first time you saw the tile. Once you're in furiten, you can only win by tsumo (self-draw) — even on different winning tiles. Riichi declaration locks you in: if you furiten after declaring riichi, you can only tsumo for the rest of the round.