This is a failure diagram from Gokyo Shumyo, Section 1, Problem 46.
I would like to ask, why doesn't Black simply play at instead? Seems dead to me, but now it is 1 am Singapore time, and I might be wrong...
I don't know why I missed this one. At the time I started this page, I was or should have been clearly awake #:-7
This results in an approach ko. If fills a liberty at a, then White at makes seki.
What happens if Black plays at in this diagram ?
This is another approach ko, but now Black takes first, and his loss is smaller if he loses the ko, so this is probably better. (please correct, unkx80, my previous contribution was terrible).
If here, then Black can cause an almost fill with a farmer's hat at the marked points and White is dead.
However, White plays diagonal attachment at and Black has to play to prevent two eyes. Now White throws in at ...
... and she has enough outside liberties to fill in the inside liberty, putting Black in oshitsubushi.