What happens or should happen in a tournament if a player unwillingly makes a mistake that leads the game to a wrong game state that is later unveiled by another player or outside watcher?

Should the game go backwards until the point the mistake was made? What if the rewind cannot be done? Should players ignore what happened and try not to make any further mistakes? Should the game restart?

Should the mistaken player be held responsible? What if he/she does not take any advantage on it? Should all players be held responsible, since no one pointed it out timely? What are the penalties?

It is all player's responsibility to ensure a legal board state; constants/passives. The board state will be rectified (the attachment discarded off the no-attachment character, etc) to satisfy any ignored constants, while missed passives are a bit trickier; the general rule of thumb appears to allow the passive to go off, provided no decisions have been made according to the board state since then (eg missing a Kings of Summer draw while you're still in marshalling doesn't hurt your opponent at all in most cases), rather than rewinding fully. A common example is people noticing a refugee surviving dominance a few phases too late; that'll typically just result in the refugee heading to the discard pile. If it's a couple actions back and no new hidden information (eg cards from hand) has been revealed, you can occasionally get away with rewinding.

There aren't any "penalties" (although the change in board state could result in one player winning), because it's everyone's job to ensure a legal board state. However, if one player does seem to be consistantly missing passives that hurt him or failing to point them out (say, your opponent's winter kingsroads), that falls squarely under a violation of the tournament rules in 'selectively' remembering/enforcing aspects of the game that benefit you. That could, at the TOs discretion, result in a disqualification.

In a nutshell:

- If it's a mutual mistake, caught relatively quickly and the incorrect board/game state can be fixed without affecting anything that has happened in the meantime, just fix it. Example: Standing the reducers in Marshalling that you forgot to stand in the last Standing phase.

- If it's a mutual mistake, caught relatively late and the incorrect board/game state can only be fixed by undoing a number of things that have happened in the meantime, fix it as best you can without "undoing" or replaying anything (or just leave it, depending on circumstances) and play things correctly from that point on. Example: Getting to the Standing Phase and realizing you could have stood a character for Vigilant in the Challenge phase. Stand it now, but too much has happened for you to reset to the point in the challenge phase where it would have stood, then replay all the challenges (and Dominance).

- If it's not a mutual mistake and one it looks like one of the players willfully misplayed in order to get an advantage, call the TO to make a ruling on what to do (which can range from "reset and replay" to disqualification).

The ultra short version is that if you and your opponent make mistakes that lead to an illegal board/game state, whatever solution you come up with that works for the two of you is the one to go with.

Oh, and during a tournament, "outside watchers" really shouldn't be offering their advice or observations on what the players are doing wrong. It's not their game, so it's not their place.

But they should notify a judge, right?

Well, I think that's more a question of how individual judges handle their events.

I personally don't see much difference between John saying, "You did that wrong" and John going to get a judge, who didn't see it, and having the judge say, "John says you did that wrong."

Generally speaking, I don't think it is the responsibility of the judge to police any game during an event. As I said above, I think the best solution is always for the players in that game to come to an agreement about what to do when a mistake is discovered. The judge is there for when they can't come to an agreement on their own and call for a ruling. But if the players don't call for a ruling, I think the judge should be careful about volunteering one that wasn't asked for. And I don't think it is ever up to a spectator to call for ruling that the players didn't ask for.

That said, if one player is taking unfair advantage of the other in a game (usually because of relative experience of just being a jerk) and the judge observes it, I think he should step in under conduct and sportsmanship considerations. And I don't think it is amiss for a spectator to call a judge over quietly to observe a game that one player seems to be playing unfairly in. But this is a lot different from a situation where Player 1 sets 6 gold or two "Limited" cards during Setup and neither player realized it until one of them has Marshalled.

Other judges may see it differently, but that's my general take.

In the tournaments I go to, it usually goes like this:

• in Swiss rounds, judges (who are usually players as well) only come when called and no player is allowed to watch another game (and anyone who knows the game is usually a player or organizer at that point)
• in tops, judges (at that point, none of them are players) actively watch the games (except in the first round or two, when there aren't enough judges to have one at each table) and non-qualified players are allowed to watch games

So when a spectator sees a mistake and points it out to the judge, the judge is right there and either goes "you're right" or "no, it's not mistake".

Thank you all for the answers.

Khudzlin's way to proceed seems the most reasonable to my taste. It is every truthful player's desire that the game proceeds as accurately as possible in regard of the rules. So it is very common, in my area, that people point out mistakes. Although, I agree outside information of any kind may be crucial to player's decisions and as such it may disrupt the flow and alter the results of the match.