phranque - 3:02 am on Aug 6, 2012 (gmt 0)

the Location: header is irrelevant in a 404 response.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.30

Location The Location response-header field is used to redirect the recipient to a location other than the Request-URI for completion of the request or identification of a new resource. For 201 (Created) responses, the Location is that of the new resource which was created by the request. For 3xx responses, the location SHOULD indicate the server's preferred URI for automatic redirection to the resource.

nothing about 4xx responses there...

http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.5

404 Not Found The server has not found anything matching the Request-URI.

you don't want to provide a location in response to a 404 - the location of the custom error page is irrelevant to the user agent or the visitor.