5xx:
These error messages are sent when the server is aware that it has had a problem or error.
500 Internal Server Error:
The server encountered something unexpected that didn't allow it to complete the request. This is often seen with CGI scripts that have problems.
501 Not Implemented:
The server doesn't support the functions required for fullfilling that request. This might occur if a server side include were called on a server that doesn't support that function.
502 Bad Gateway:
The server, while acting as a gateway or proxy, received a bad request from an upstream server.
503 Service Unavailable:
The server is unable to handle the request due to maintenance or a temporary overload of the server.
504 Gateway Timeout:
The server, while acting as a gateway or proxy, did not receive a timely response from an upstream server.
505 HTTP Version Not Supported:
The server does not support the HTTP version that was used to make the request.
