one of the documented procedures in this installation of the ACS
ug_group_page_does_not_exist   db   group_id   group_name   group_admin_email   page_name   link_target   link_name
What it does:
this procedures return message to the user that a page does not exist. the page display is customized for group pages and uses ug_header and ug_footer. user is redirected to appropriate page with name link_name and link target link_target
Defined in: /web/philip/tcl/user-group-defs.tcl

Source code:

    set page_title "Page does not exist"
    ns_return 200 text/html "
    [ug_header $page_title $db $group_id]
    [ug_page_title $page_title $db $group_id $group_name]

    <h4>Page $page_name does not exist.</h4>
    <h4><a href=\"$link_target\">$link_name</a></h4>

    [ug_footer $group_admin_email]