Chombo
Chombo (冲合) is a penalty to players performing illegal moves. Some moves may be considered to be minor, like accidentally drawing the wrong tile from the wall. Penalties subject to chombo are harsh enough to warrant point losses to the offender. The penalty for chombo requires the offender to pay the amount of mangan.
Cheating
Cheating is very possible in the game. In fact, there are known various and elaborate cheating tricks. Among these include stealing from the discard pile, or tile trading from the hand to the wall. For example, some tricks involve the sleight of hand.
Invalid win
Players cannot declare a win with an incomplete hand or incorrect waiting tile.
Win with no yaku
A hand must have at least one yaku minimum to win. Therefore, declaring a win on a hand without yaku is penalized.
Win during furiten
A player in furiten cannot win by ron. Instead, by the rule of furiten, a player is limited to winning via tsumo. Declaring ron while furiten is penalized.
Wall destruction
A player cannot crash and destroy the wall, such that the tile arrangement cannot be recovered. In fact, this act may be considered to be outright rude, if done intentionally.
Riichi based
Riichi has some very specific rules and breaking these rules may be grounds for chombo.
Noten riichi
Riichi must be declared with a tenpai hand. Therefore, riichi declared with a noten hand is penalized.
Unrevealed riichi
If the hand ends in ryuukyoku while at least one player declared riichi, it is a requirement for the riichi hands to be shown, as proof of being in tenpai. Yet, a player may actually refuse to show the hand and instead take the penalty.
Invalid ankan
During riichi, a player may call kan with an ankan, or closed kan. However, this is invalid when the player's wait changes.