400 Bad Request
The server cannot or will not process the request due to something that is perceived to be a client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing).

401 Unauthorized
Although the HTTP standard specifies "unauthorized", semantically this response means "unauthenticated". That is, the client must authenticate itself to get the requested response.

500 Internal Server Error
The server has encountered a situation it does not know how to handle. This error is generic, indicating that the server cannot find a more appropriate 5XX status code to respond with.