The 4xx class of status code appears when a client made a mistake. Except when responding to a HEAD request, the server SHOULD include an entity containing an explanation of the error situation. It is whether a temporary condition, whether a 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 the 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 TCP stack will send a reset packet to the client, which may erase the client unacknowledged input buffers before they can be read and interpreted by the HTTP application.