404 Not Found
The server has not found anything matching the requested URL. This file doesn't exist. What you get if you give a bad URL to your browser. This can also be sent if the server has been told to protect the document by telling unauthorized people that it doesn't exist. 404 errors are the result of requests for pages which do not exist, and can come from a mistyped URL, a bookmark which points to a file no longer there, search engines looking for a robots.txt (which is used to mark pages you don't want indexed by search engines), people guessing filenames, bad links from your site or other sites, etc.