Tsumego elementary problem 157
Black lives:

$$B 
$$ -------------------
$$ | . 1 . . X O . . .
$$ | O X . X X O . . .
$$ | . X X O O O . . .
$$ | O O O . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .


White kills unconditionally:

$$W 
$$ -------------------
$$ | . 2 1 . X O . . .
$$ | O X . X X O . . .
$$ | 3 X X O O O . . .
$$ | O O O . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .


This doesn't work for Black, either.

$$W 
$$ -------------------
$$ | . . 1 . X O . . .
$$ | O X 3 X X O . . .
$$ | 2 X X O O O . . .
$$ | O O O . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .