10.5.4 503 Service Unavailable
The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response.
10.5.1 500 Internal Server Error
The server encountered an unexpected condition which prevented it from fulfilling the request.
Što če reći, spor i nestabilan server.