HTTP 404 Error Page

Some websites report a "not found" error by returning a standard web page with a "200 OK" response code; this is known as a st 404. St 404s are problematic for automated methods discovering whether a link is broken. Some search engines, like Yahoo, use automated processes to detect st 404s. St 404s can occur as a result configuration errors when using certain HTTP server stware, for example with the Apache stware, when an Error Document 404 (specified in a .htaccess file) is specified as an absolute path (e.g. http://example.com/error.html) rather than a relative path (/error.html).
Some proxy servers generate a 404 error when the remote host is not present, rather than returning the correct 500-range code when errors such as hostname resolution failures or refused TCP connections prevent the proxy server from satisfying the request. This can confuse programs that expect and act on specific responses, as they can no longer easily distinguish between an absent web server and a missing web page on a web server that is present.
In July 2004, the UK telecom provider BT Group deployed the Cleanfeed content blocking system, which returns a 404 error to any request for content identified as potentially illegal by the Internet Watch Foundation. Other ISPs return a HTTP 403 "forbidden" error in the same circumstances. The practice employing fake 404 errors as a means to conceal censorship has also been reported in Thailand and Tunisia. In Tunisia, where censorship is reportedly severe, people have become aware the nature the fake 404 errors and have created an imaginary character named "Ammar 404" who represents "the invisible censor".