Describe OneThousandAndOneLifeAndDeathProblems/Erratum12 here.
The book explicitly states that there are no problems that need a ko to solve, but in this one the answer doesn't mention a move by white that forces a ko!
The solution given in the book:
But if white plays at
, white can make a ko! Watch:
And now we have a two-step ko for white, which, if won, kills black.