Http status code 500 series

5xx Server Error
The server failed to fulfill an apparently valid request.
Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has encountered an error or is otherwise incapable of performing the request. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and indicate whether it is a temporary or permanent condition. Likewise, user agents should display any included entity to the user. These response codes are applicable to any request method.

500 Internal Server Error
A generic error message, given when no more specific message is suitable.

501 Not Implemented
The server either does not recognise the request method, or it lacks the ability to fulfill the request.

502 Bad Gateway
The server was acting as a gateway or proxy and received an invalid response from the upstream server.

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

504 Gateway Timeout
The server was acting as a gateway or proxy and did not receive a timely response from the upstream server.

505 HTTP Version Not Supported
The server does not support the HTTP protocol version used in the request.

506 Variant Also Negotiates (RFC 2295)
Transparent content negotiation for the request results in a circular reference.

507 Insufficient Storage (WebDAV) (RFC 4918)

509 Bandwidth Limit Exceeded (Apache bw/limited extension)
This status code, while used by many servers, is not specified in any RFCs.

510 Not Extended (RFC 2774)
Further extensions to the request are required for the server to fulfill it.