Problem diagram.
Black's response to White's marked play?

Source:
Go World, #35, Spring 1984, p. 56
Ohira Shuzo vs. Sugiuchi Masao
9th Gosei, 3rd prelim. round.

Solution follows    (or click here to download SGF)

Horrible.
Shuzo makes a fatal tsumego mistake.

At first glance, the throw-in with 1 looks like the right move. But even professionals should double check. After White 4 (at 1) Black resigned. The semeai is one move short for Black.

Shuzo said that this was the first time in his life that he had made this kind of tsumego mistake. I know how Shuzo feels; I cannot remember the last time I made a tsumego mistake. Right.

Correct.
Black 1 is the move. Now the throw-in at 3 works. Black 5 wins the semeai.

Second best.
Even Black 1 here, which only leads to a sente seki, would have been better.