Countering mirror go
Mirror go played by White can be countered by Black playing on the tengen, although this is not necessarily the best way. Below are several examples of countering mirror go. The result is often absurd, but if the player who is mimicking aborts halfway, the other player would have succeeded in stopping the mirror go.
The more challenging task is to do so without giving your opponent any opportunity to take advantage of your mirror defense attempt (by suddenly responding with a better move than yours).
1. Shortage of liberties due to tengen
What's the best continuation if connects at 9?
2. Setting up two ladders
Incidentally, this very technique is seen in a professional game: Hosai Masao - Two Ladders Mirror Go.
Here's another standard  anti-mane-go strategy that can be used instead. It assumes that some form of superko rule is in force. White starts by creating two kos; call these kos A and B, and their mirror images A' and B'. White captures in A, and Black mimics by capturing in A'. Then White captures in B, and Black captures in B'. Then White recaptures in A', and Black recaptures in A. Then White recaptures in B'. The superko rule forbids Black from recapturing in B, so he has to stop mimicking.
Not everything that looks like it, is actually mirror go. In a database search, there are nine pro games with this exact position. White won eight out of nine, and seven of those were without komi. Since 1985, the pros seem to have decided that this is not a viable strategy for black. (If you delete from the board and look at it as black to play, it's quite a common position--and people don't often play tengen here.)
It would be a mistake is to play an irrational move in an attempt to capture black in a cheap trick. In Example 2, white notices black's mirror go when black plays . White plays in an attempt to begin a capturing race in which is a burden to black. However, when black breaks mirror go by extending to a. Ladder strategies may share of the same bad taste.
One may still feel this is playable for White.
Proper strategy is to recognize the possibility of mirror go from the start and play a solid territorial game in which black's tengen stone feels inconsistent and does not aid black in fighting or building a moyo. In Example 3, white plays a solid territorial game that would feel like relying on komi even if black were not playing manego. While is a natural move, makes feel like a light refutation of ippoji. Black has played a territorial game that is inconsistent with his first move and has lost most of its advantage.
xela: What if white plays and then black decides not to play mirror go? Many people would prefer to have on the 4-4 or 4-5 point.
- The game Kato(w) versus Takemiya(b) played in the Meijin League in 1995 began with mirror play. After seven moves the position resulted as shown in the diagram:
Takemiya played , which would be considered soft shape, thinking that if the moves white a through black f followed the position would result with the black stone on the tengen would be developing Black's moyo while erasing White's moyo, as shown in the following diagram, so Kato would break off mirror play. In the game, Kato immediately broke off mirror play by playing at the circled point approaching the lower right corner.
- Karami has a pro example. I don't know if it was mirror go, but when Black finally played on tengen, the position was symmetrical. And the move was decisive. See http://gobase.org/replay/?gam=/games/japan/titles/minister/17/game-21.sgf at GoBase. Upto that tengen play, the game indeed was some interesting kind of mirror Go, that is, not move wise, but joseki wise.
 Robert Jasiek: For a strategy invented by me during the late 90s, I am impressed that it is already called "standard". Actually, the strategy also involves creation of kos of different values and winning the bigger ko.