The description on the Disallow usernames page of a board's Admin Panel states "Please note that you will not be allowed to specify any username that has already been registered, you must first delete that name then disallow it." This is not true. If "tester_2" is registered, that username can still be disallowed. There is no check performed against registered usernames in acp_disallow.php.
The description is stored as ACP_DISALLOW_EXPLAIN in language/en/acp/posting.php. It should probably be altered in a future version.