401 Unauthorized

Similar to 403 Forbidden, but specifically for use when authentication is possible but has failed or not yet been provided. The response must include a WWW-Authenticate header field containing a challenge applicable to the requested resource.

403 Forbidden

The request was a legal request, but the server is refusing to respond to it. Unlike a 401 Unauthorized response, authenticating will make no difference

404 Not Found
The requested resource could not be found but may be available again in the future.Subsequent requests by the client are permissible.

500 Internal Server Error

A generic error message, given when no more specific message is suitable

503 Service Unavailable

The server is currently unavailable (because it is overloaded or down for maintenance). Generally, this is a temporary state.

509 Bandwidth Limit Exceeded

Apache bw/limited extension
This status code, while used by many servers, is not specified in any RFCs