Furiten

Furiten is a Japanese Riichi restriction that forbids winning by ron (claiming a discard) when any of your waiting tiles already appears in your own discard pile. The logic is that you passed up that winning tile earlier, so you cannot suddenly claim it from an opponent. While in furiten you may still win, but only by tsumo (self-draw); you simply cannot take the win off another player's discard.

The rule checks every tile in your wait, not just the one most recently discarded. If your hand waits on 3p or 6p and even a single 6p sits among your past discards, you are furiten on the entire wait, blocking ron on both 3p and 6p. This is why wide multi-sided waits, while otherwise excellent, carry extra furiten danger: more accepted tiles means more chances one of them was already thrown.

There are two further forms. Temporary furiten arises when an opponent discards your winning tile and you decline it; you are locked out of ron until your next draw, after which it clears (unless you are in permanent furiten). Riichi furiten is permanent: if you pass on a winning tile after declaring riichi, you are furiten for the rest of that hand and can only win by self-draw.

Furiten exists only in Riichi and similar Japanese-derived rules. Chinese Official, Hong Kong, and Taiwanese mahjong generally have no equivalent and let you win off a previously discarded tile freely. Because of this, furiten awareness is a core Riichi skill: before declaring riichi, strong players verify their entire wait against their discards, since locking into a furiten riichi can leave them only able to win by drawing the tile themselves.