500 Internal Server Error: This is a generic error message that means something went wrong on the server, but the server can't be specific about the exact problem.

501 Not Implemented: The server running the site does not understand or does not support the HTTP method sent by the client.

502 Bad Gateway: This error is usually due to improperly configured proxy servers.

503 Service Unavailable: The server is currently unable to handle the request. This may be due to temporary server overload or maintenance.

504 Gateway Timeout: The server did not receive a timely response from another server that it needed to fulfill the request.