Why is this considered a mistake?
Korlon: There are a number of responses to this after black cuts. This move 4 by white is not joseki because, unless white has neighboring stones around, perfect play by black should lead to him getting a better result.
Sodavich: I have a lot of trouble punishing white's mistake. They say that only beginners rush to atari. What if white plays like this?
unkx80: If atari, then extends and the corner stones are dead.