Staying on the topic of fails on my part, here is one of my games from EGC 2016. I played a bad variation, but knew exactly how many points it was worth.
Inmidst the centre close combat, I decided that making the seki in upper left was the next biggest move on the board. Seki sequence yada-yada…
How should Black play next?
Evidently, the sequence isn’t finished yet; if Black tenukies, White can play A-B-C to capture Black. Therefore, I played 1-3 hanetsugi to make it a seki. But it turns out that this hanetsugi is a careless move 隨手 súi shǒu.
Even though this is the sequence I had in mind when I started it, White got sente for the massive move in the centre.
Black should donate another stone by crawling at 1! This way, the corner is already seki and it’s Black’s sente for the next move in the centre.
I saw this move immediately after playing hane (even before tsugi). What a blunder I made here! To top it off, my opponent was completely ignorant of the fact that I made a mistake here. This move instantly spoiled my mood and crushed my fighting spirit. And thus I resigned some moves thereafter.
While my opponent was thinking for his moves, I had time to calculate how many points my hanetsugi-sequence made. If I had donated one more stone for sente, I would have lost the marked point and White would have captured two more stones. (Eventually Black will need to throw in after White takes two stones.) That means, in the game I made 5 points by spending one move. Having established that a reverse sente is worth exactly double the face value, the hanetsugi is equivalent to playing 10 points gote. That of course cannot justify giving up the next move in the centre, which is of immesurable value.
Even though knowing this number is more or less useless, donating one more stone is a simple, yet effective trick that you can learn and use in your game.
That said, I made another fail earlier in the game when I missed a sente sequence and let my opponent reverse it:
I finished the ko (there was a white stone inside at j16) and White took the chance to atari and tiger in upper right. This move prevents Black from capturing the corner stones in sente by playing double-hane and donating one ponnuki.
Furthermore, White can keima out at A next. In the game I blocked it in a way that gave White a prisoner and left him a Damokles-ko in sente (see previous diagrammes). We will summarise the value of White’s sente continuation in Ɛ, given that Ɛ>0.
Since the ko does not affect the safety of any group and taking that stone only augments the centre influence, Black definitely had time to insert the picture double-hane sequence. (Basically White has to accept the ponnuki. If White connects back, Black connects outside in sente and White has only 3 points in the corner.) White needs to spend a move around 8 to prevent Black’s A, so Black ends in sente and can still go back to take at 9.
In the diagramme I marked the territory Black gains: 11 points. White made 6 points in the game; here White has around 4 while Black can choose to descend at t10 later (which affects the s2 endgame) or to harass that group with A. So this sequence would have been around 13+Ɛ points in sente, and in the game I unnecessarily let White reverse a juicy 26+2Ɛ point-move.