According to wikipedia, "when communicating via HTTP, a server is required to respond to a request, such as a web browser's request for a web page, with a numeric response code and an optional, mandatory, or disallowed (based upon the status code) message. In the code 404, the first digit indicates a client error, such as a mistyped Uniform Resource Locator (URL). The following two digits indicate the specific error encountered. HTTP's use of three-digit codes is similar to the use of such codes in earlier protocols such as FTP and NNTP."
That's the techspeak, but most of us recognise the 404 page as the result of an annoying broken link or a mistyped URL - something that nine times out of ten is incredibly annoying.
But that's not the case with these clever, inventive beauties. Turning a negative into a positive, it's almost worth getting things wrong to get one of these to pop up.
Take a look at our favourite 11 in the gallery below.