hmm.
you could :

1. check your web server's log files. You don't say what flavor server you're using, but it might be possible to log the url that generates this error.
2. whomp up a little LWP::Useragent script to hit the page and redirect repeatedly, and alert you when such an error occurs.
3. uh,use strict;
4. if your users are in the same building/town/area as you, have one of them call you when the error occurs. then run! to them, and view the error for yourself.

obviously, I must advocate the middle two for their Obperlishness, but the first and the last have their own piquant qualities as well.