Here is an odd case, shared by James Davies in 1996 (source):
Black to play, no komi. What’s the result?
If black adds a move to capture the white stones, both sides have 28 points, and the result is jigo.
Black would like to avoid adding a move, to gain a point and win the game.
So black considers what would happen if he passes and white plays first inside:
White can exchange 1 for 2, but after that it is clear that black is safe. The triangle is a real eye for black because of the double ko. Black has two eyes, white has one eye, so white is dead. Indeed this is true even under the special ko-rules used for status determination after the game is finished, so both sides can pass here and the white stones can be removed without further moves. Black wins by one point.
However, white had a stronger response! Instead of exchanging 1-2 above, she should pass immediately, ending the game. Then she can demonstrate that the black stones are dead as follows:
The exchange 1-2 is still forcing like before. But now when white captures at 3, black is not allowed to recapture 1 without first passing for that ko. Black has no way to stop white from capturing at A with the next move, and then at B with the move after that.
The conclusion is that black cannot pass in the original position, but must add a move and settle for jigo.
What makes this an odd case (in my opinion) is that:
- In actual play, black is clearly alive in double ko.
- Even though the 1-2 exchange is always the obvious start of the sequence, it makes a difference if it is played before or after the game end. If white makes this exchange before ending the game, then suddenly the black group is alive without needing an extra move.
Left board: black is dead. Right board: black is alive.
