Game Play Errors in Depth

A GRV covers a player breaking the rules of the game in a way not covered by a more specific infraction — playing a land twice, attacking with a tapped creature, resolving something incorrectly. Default penalty: Warning. The judge then applies the prescribed fix, which usually means leaving the game as-is or a limited backup if it was caught immediately.

When one player commits a GRV, the opponent shares responsibility for not catching it. If the opponent allowed the illegal game state to occur (and it isn't reasonable that they missed it), they receive Failure to Maintain Game State — also a Warning. This reflects that both players are responsible for a legal game.

A triggered ability is "missed" when a player doesn't acknowledge it at the appropriate time. Key points:

• A player is not forced to remember an opponent's triggers; only its controller is responsible for their own triggers.
• Default penalty is no penalty for missing a beneficial trigger (with a Warning in some cases).
• If a missed trigger is detrimental to its controller (e.g. "sacrifice a creature"), the opponent chooses whether to resolve it when it's discovered.
• Whether a missed trigger can still be put on the stack depends on how much time has passed (roughly, before a player takes an action that a change would affect).

• Looking at Extra Cards — a player briefly sees a card they shouldn't (e.g. flips one off the top). Penalty: Warning; the seen card is usually shuffled back into the appropriate part of the library.
• Hidden Card Error — a player illegally puts a card into a hidden zone or has the wrong number of hidden cards (e.g. drew too many). Penalty: Warning; a randomization/reveal fix is applied.
• Mulligan Procedure Error — a mistake in the mulligan process. Penalty: Warning, with a fix depending on the stage of the mulligan.