HTTP 404 Error Page

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".
In 2008, a study found that "404" had become a slang synonym for "clueless" in the UK/Ireland areas. Slang lexicographer Jonathon Green said that "404" as a slang term had been driven by the "influence technology" and young people, but at the current time, such usage was relatively confined to London and other urban areas. The study, carried out by the telecommunications arm the Post Office, concluded that writing abbreviations such as "404" was expedient to the sender, while an Australian study found that such phrases impaired the intelligibility the message.