To replicate:

1. Find a page that requires a login to access
2. View the page attachments and copy the URL (should be something like: http://confluence.atlassian.com/pages/viewpageattachments.action?pageId=12345)
3. Log out, and return to that URL

You should be presented with "Page Not Found", when a more appropriate response would be asking for a login - or [if logged in] telling them that they dont have the required security access (as that page id does exist, it just is not accessible with your current security privs).