This error means the specified page was not found, but the website responsible returned a friendly error page.
Note that some badly behaved websites will return a ‘Page not found’ (404) error even when a page is found. If this is the case, you will see this error here. Such websites will experience various side effects, most seriously many will be inaccessible behind certain proxies or from some ISPs.
Compare with Page not found (ungraceful).
Wikipedia article on 404 errors.