how illegal moves are handled
All rulesets contain rules, therefore ought to contain a meta-rule consisting of what do do when these rules are not followed. What to do if this meta-rule is broken is rarely (never?) considered, but to avoid an infinite series of meta-rules, the meta-rule ought to be able to be applied to itself.
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The AGA rule states:
Illegal Moves: An illegal move is one violating the rules. If a player makes an illegal move--such as moving twice in a row (i.e., before the opponent has made a response), attempting to play on an occupied intersection, self-capture, or retaking a ko so as to repeat the full board position, the player must take back his or her move (both moves, if he or she moved twice in succession), it shall be treated as a pass, and a pass stone exchanged.
An illegal move must be noted as such by the opponent before he or she makes his or her move. When a player moves, he or she is tacitly accepting the opponent's previous move as valid. In particular, if it is discovered that an earlier move by one of the players was illegal, the game must nevertheless be continued as it stands unless both players agree to restore the earlier board position and proceed from that point.
I think we know what they mean, but the final sentence should probably read "In particular, if it is discovered that an earlier move by one of the players was illegal and not noted as such , the game must nevertheless be continued as it stands unless both players agree to restore the earlier board position and proceed from that point." To avoid claims that any illegal move must stand.
Do these rules allow the following sequence?
Black does not notice that 1 is illegal. Black plays elsewhere. Anon White could claim that Black's move elsewhere was illegal, since Black failed to remove . Black would then have to pass instead.
White plays 1, capturing the 6 black stones.
Note that if Black noticed that white 1 (in the first diagram) was illegal, the stone would be removed and white would be considered to pass. If there are no point scoring plays left, white loses nothing and potentially gains a lot. A good tactic? Possibly a useful ruse when playing somebody who continues to play after the game is over, or a huge number of points behind. Usefully, such play could not continue for long, as two passes ends the game.
Article 14. Forfeit
Violation of the above rules causes immediate loss of the game, provided the result has not yet been confirmed by both players.
What if the violation is not noticed immediately? Immediate loss of the game for which player?
Chinese Rules ( according to James Davies)
Chapter 3. Section 19. Forbidden points
If a stone is played on a forbidden point, the move is declared invalid and the player loses his turn.
A forbidden point is earlier described as one that would cause self-capture. Again, what happens if nobody notices is not described, nor are the many other possible rules violations considered.
No mention of rules violations in the New Zealand Rules?