Tsumego From Games 2

Black to live  

Already, the - exchange was not so good. But Black can still live.

Wrong  

With , White had envisioned the following sequence: Black takes two stones, and White recaptures at , making miai of a and b, killing the corner. But he overlooked a black alternative.

Correct  

If Black doesn't capture the two stones, he leaves them in shortage of liberties. Now he can play at . prevents Black making two eyes, and Black plays atari at . White can't cover at a because of the existing shortage of liberties. In the failure diagram, which was also the game, Black himself resolves White's shortage of liberties, and thereby kills his own stones.

Status ?  

Black thought he could kill White. He cannot.