2 Answers

The 404 or Not Found error message is a HTTP standard response code indicating that the clientwas able to communicate with the server, but the server could not find what was requested.The web site hosting server will typically generate "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 users can find on the web.[1]

A 404 error should not be confused with "server not found" or similar errors, in which a connection to the destination server could not be made at all. A 404 error indicates that the requested resource may be available again in the future; however, the fact does not guarantee the same content.

Wiki

On this site' I've had it appear if I try to go back to a question that was asked, but has subsequently been deleted.