The 4xx class of status code is intended for cases in which the client
seems to have erred. Except when responding to a HEAD request, the server
SHOULD include an entity containing an explanation of the error situation,
and whether it is a temporary or permanent condition. These status codes
are applicable to any request method. User agents SHOULD display any included
entity to the user.
If the client is sending data, a server implementation using TCP SHOULD
be careful to ensure that the client acknowledges receipt of the packet(s)
containing the response, before the server closes the input connection.
If the client continues sending data to the server after the close, the
server's TCP stack will send a reset packet to the client, which may erase
the client's unacknowledged input buffers before they can be read and
interpreted by the HTTP application.
The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications.