Why do I get a URL "not found" or "moved permanently" error after applying a patch/upgrade?

If you are experiencing a URL "NOT FOUND" or "MOVED PERMANENTLY" error after applying a patch or upgrade, then set Apache ProxyPreserveHost token to ON in the httpd.conf file.

If you have applied a patch or upgrade and are now receiving the following error:

<The document has moved <a href="https://www.<site>/sf/global/jsp/buildtime.html" 
    format="html" scope="external">here</a>.</p>
<hr> <address>Apache/2.2.3 (Red Hat) Server at www.<site>.com Port 80</address> </body></html>
Not Found 

The requested URL /sf/sfmain/do/userPicker/projects.pftool//sfmain/do/listMonitoringUsers/projects.pftool/discussion.announcements was not found on this server

Or if you are trying to add users to a monitoring list, and are receiving the following error:

Not Found
The requested URL 
    /sf/sfmain/do/userPicker/projects.pftool//sfmain/do/listMonitoringUsers/projects.pftool/discussion.announcements 
    was not found on this server.        

Set the ProxyPreserveHost token to ON in the httpd.conf file.