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In Go, there is a potentially repetitive sequence of moves called a "ko." So if your opponent takes a ko from you, you have to make a move elsewhere on the board, called a ko threat, before "taking back" the ko, in order to break up the repetition. But suppose someone "repeats" by taking back a ko, without making a ko threat. What are the penalties?

My understanding is that in a tournament, the penalty will be a forfeit of the game. But I've never seen this rule enforced in informal play. The standard "fix" is that the offender is forced to take back his ko "recapture," and make another move.

Is my understanding of the tournament rule correct? And is there an informal "house rule" that prevents the application of this rule in casual games?

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One of the situations where online go is nice. Can't make that mistake :) – CodesInChaos Jul 21 '13 at 18:38
@CodesInChaos Even online you still can make that mistake, you just can't click. After trying to click several times, you will realize it: "oh, why can't I play there? oups!" It's just that your opponent doesn't know about it... Actually, I would prefer an online server where you can make such a move, and if you do, you lose immediately. – Djaian Jul 22 '13 at 19:52
@Djaian At least on KGS the point(s) blocked by ko are marked with a rectangle. – CodesInChaos Jul 22 '13 at 20:04
@CodesInChaos I know. But when you are thinking, you might still want to play there, and realize your mistake only when you click. – Djaian Jul 22 '13 at 20:06
up vote 6 down vote accepted

At least in European tournaments current rule is that if you play an illegal move you have to take it back, and play a legal move. You don't lose the game by default.

Illegal move

If a player makes an illegal move, and if this is noticed within three moves, then the game should be unwound to the move just before the illegal move, and continued. The referee may allow an adjustment of the time.

EGF general tournament rules

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This is reasonable! – Tomas Nov 11 '14 at 14:49
AGA rules treat it as a pass: "Illegal Moves: An illegal move is one violating the rules. If a player makes an illegal move, it shall be taken back, treated as a pass, and a pass stone exchanged." – CodesInChaos Feb 28 '15 at 19:33

In all official rules that I know this means instant forfeit of the game. Of course, in casual play, mistakes like this can be allowed to take back at the will of the players.

In tournaments I would strongly advise against making such a mistake as well as trying to ask for a take back. If the other player does not seem to allow a take back by his own active will, just resign and promise yourself to not make the same mistake again. Mental strength is part of the game, too, after all (just like time management, i.e. losing on time).

The reason players sometime forget to make a threat is often that after player A took the ko, player B takes some time to contemplate the threats, the board, counts the score, and finally simply recaptures the ko - the need to play a threat slipped out of their mind in the meantime.

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Apparently this isn't true for the official EGF (taken back if noticed within three moves) and AGA rules (taken back, treated as a pass). – CodesInChaos Feb 28 '15 at 19:32
I was not aware of that. I thought the rules we used at the EGC (and other tournaments) are official, and did not cover this - or did they? I may have misremembered. – mafu Feb 28 '15 at 21:55

One important point to understand is that in an amateur tournament, whenever both players agree on something, then it's ok. For example, if one player plays an impossible move (like taking back a ko immediately) and the other says "take back your move and let's pretend nothing happened", then so it is: nothing happened.

Problems only occur when the players disagree. According to the rule, playing an impossible move is an immediat loss. That means, the other player can, if he wants, declare victory.

Usually weaker players will easily let their opponent take their move back. When it comes to stronger player, they tend to claim they win if the opponent makes such a mistake.

The worst that can happen is that someone else, not playing but just watching interfer. As a tournament organizer I hate when people interfer in other people's games. That's usually when the fuss starts. The player agreed to let his opponent take back, but someone watching tells he should claim victory, and so on.

In a professional tournament the player making the mistake will resign as soon as he sees it (such mistakes are very rare in professional tournaments, but may happen sometimes).

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I have seen what you're describing in tournaments, too. A player made a mistake and took the ko when it was illegal, and the other player just made him take the move back.

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This doesn’t answer the question:( – PJTraill Jun 11 at 11:38

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