Ron decline into temp. furiten
Most commonly, a tenpai hand is in furiten if any of that player's winning tiles are present in one's own discard pile which includes called tiles, or another player's discard pile during a turn or after its own riichi. Furiten hands are still winnable via self-draw. A call of ron while furiten is penalized by chombo, though computer games are programmed to prevent the player from declaring such a win.
In all cases, the furiten rule requires a player to be aware of all of their tile waits. Furiten applies to all waiting tiles, even that to a thirteen sided kokushi musou. If a player is tenpai but does not realize all of their possible waits, then the lack of awareness may lead to an illegal win. This is especially true of a player who has called riichi and end up passing on a winning tile.
To enforce furiten, the discard pile is specifically arranged in front of players and tile calls are still noted as part of discard piles as turned tiles. The discard arrangements allow players to keep track of who discarded which tiles. During a hand, players should take note of their status in relation to furiten and avoid calling winning hands while in furiten. Likewise, any calls for winning hands should have the discards checked for furiten. Any game involving software already checks for furiten, alerts players, and denies winning calls.