one of the documented procedures in this installation of the ACS
ug_page_does_not_exist   page_name   link_target   link_name
What it does:
this procedures return message to the user that a page does not exist. it uses ad_header and ad_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 "
    [ad_header $page_title]

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