HTTP 404

[1] The website hosting server will typically generate a "404 Not Found" web page when a user attempts to follow a broken or dead link; hence the 404 error is one of the most recognizable errors encountered on the World Wide Web.

When communicating via HTTP, a server is required to respond to a request, such as a web browser request for a web page, with a numeric response code and an optional, mandatory, or disallowed (based upon the status code) message.

In code 404, the first digit indicates a client error, such as a mistyped Uniform Resource Locator (URL).

[4] This can also be done on purpose to force some browsers (like Internet Explorer) to display a customized 404 error message rather than replacing what is served with a browser-specific "friendly" error message (in Internet Explorer, this behavior is triggered when a 404 is served and the received HTML is shorter than a certain length, and can be manually disabled by the user).

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.

[6] The practice of employing fake 404 errors as a means to conceal censorship has also been reported in Thailand[7] and Tunisia.

[8] In Tunisia, where censorship was severe before the 2011 revolution, people became aware of the nature of the fake 404 errors and created an imaginary character named "Ammar 404" who represents "the invisible censor".

For example, Metro UK shows a polar bear on a skateboard, and the web development agency Left Logic has a simple drawing program.

[15] In Europe, the NotFound project, created by multiple European organizations including Missing Children Europe and Child Focus, encourages site operators to add a snippet of code to serve customized 404 error pages[16] which provide data about missing children.

[18] A number of tools exist that crawl through a website to find pages that return 404 status codes.