I just noticed that when a user looks at the list of member groups in someone's profile, he will generally not see hidden groups even if the user who is doing the looking is a member of the hidden group. I assume the list should actually include hidden groups of which the viewing user is a member, correct?

The problem is in this code in memberlist.php:

		// Do the SQL thang

		$sql = 'SELECT g.group_id, g.group_name, g.group_type

			FROM ' . GROUPS_TABLE . ' g, ' . USER_GROUP_TABLE . " ug

			WHERE ug.user_id = $user_id

				AND g.group_id = ug.group_id" . ((!$auth->acl_gets('a_group', 'a_groupadd', 'a_groupdel')) ? ' AND g.group_type <> ' . GROUP_HIDDEN : '') . '

				AND ug.user_pending = 0

			ORDER BY g.group_type, g.group_name';

		$result = $db->sql_query($sql);

This looks like a bug to me, but perhaps you intended it because getting it to list the hidden groups the viewer is a member of might take an extra query or at least a query that takes longer to execute?