510 Not Extended
HTTP response status code 510 Not Extended is a server error used to indicate that an extended HTTP request (in the context of RFC 2774) is required, but is not used.
When the 510 Not Extended status code is received, it is being used as part of the HTTP extension mechanism that is provided by RFC 2774. As of February 2000, this is an experimental HTTP protocol and has not been officially adopted as a standard.
The error occurs when the client is required to use an “extended HTTP request", but has not done so. Included as part of the HTTP response, the server informs the client as to how to extend the HTTP request. However, a format for doing is unspecified in the HTTP specification.
Apache HttpComponents Core
The 510 Not Extended status code is a server error that indicates an extended HTTP request is not supplied by the client, yet one is required. Consequently, the HTTP request cannot be completed.