Wikimedia Bugzilla is closed!

If the API is used to attempt to login, but the password used to do so is a temporary reset password, $loginForm->authenticateUserData() will return LoginForm :: RESET_PASS. The API does not handle this and falls through to the default case, returning an error. 

I would expect it to return a specific failure code that would indicate the use of a reset password, as suggested by the comments in SpecialUserlogin.php:

// At this point we just return an appropriate code indicating
// that the UI should show a password reset form; bot inter-
// faces etc will probably just fail cleanly here.

If it does not return a new value, it could return WrongPass, since it is not the right (normal) password.

(Yes, I actually got this one, testing MediaWikiAuth.)