ANSWER:

It seems like either your application is sending users an error page, or that you are getting random requests for a non-existent page to your web server.

You really can’t do much about non-existent pages, but if the error page is being served correctly, and doesn’t exist, that could be the problem.

You may want to examine your Apache configuration files to inspect any directives to the ErrorDocument and see what they are pointing to.

by Mike Fiedler from http://serverfault.com/questions/386524