Drupal Site Configuration: Page Not Found Errors
Sometimes visitors to your site attempt to browse to a nonexistent page. Instead of displaying an ugly page that says Page Not Found, by default Drupal sends the visitor to a special page that reports this error 404 error.
If you have pages on your site that only logged-in users can see, and unregistered visitors try to go to one of those pages, Drupal redirects them to a page-not-found page.
The settings that control this behavior are found at Configuration→Site Information→Error Pages.
On this form, you can change the pages to which visitors are sent when they try to browse to a page that doesn’t exist — or when an unregistered visitor tries to browse to a page that only logged-in users can see.
If you’re curious, you can see what pages your users are trying and failing to reach. To see logged error messages, choose Reports→Recent Log Messages. Clicking Filter Log Messages allows you to sort through the messages and locate precisely the ones you’re interested in. On the left are the types of errors; on the right are the levels of severity.