was asking to Howard at: http://moodle.org/mod/forum/discuss.php?d=131784 and, when looking at the db/auth.php plugin... I've detected that it has one (two in fact) ANNOYING print_error() if the login/password check fails.
Other auth plugins haven't that at all. Expected behaviour is to return true/false only (without own printing and die) afaik. Can you imagine any reason for that? Seems that it's there since the beginning.