Anecdotally I know of one very popular application that has a bug in their SHA-1 implementation causing it to effectively only perform a single round.
In this specific case, it's unclear whether the bug has direct security implications. The broken SHA-1 is used on some user-controlled data that gets XORed onto the server's decryption of a user-specified payload before being passed into an RC4 key schedule. It's certainly plausible that this might produce a server-assisted privacy compromise of other users' sessions.