Also if "Limit username chars" is set to "Any character" and you disallow a username (e.g. admin) you get a USERNAME_DISALLOWED error if you try to disallow a new username that begins with one you already disallowed (e.g. admins, adminis, admintra).
Linked ticket with changeset: r5694
Action performed by Anonymous (I am too lazy to register) on Mar 22nd 2006, 10:17