Sysadmin reported that the siberian logs were full of things like this:
[Sun Mar 06 00:00:00 2011] [notice] cannot use a full URL in a 401 ErrorDocument directive --- ignoring!
and it was happening a few thousand times a day.

I edited the htaccess file so that it didn't look like this:
ErrorDocument 401 http://www.siberianexpedition.ca/progress.php
I edited it to look like this:
ErrorDocument 401 /progress.php

Note that the initial / is crucial.