OS X Contacts (Mountain Lion) with CardDav - "CoreDAVHTTPStatusErrorDomain error 500"
The response says the the error is when communicating with the 3rd party server (since the Mac isn't CentOS obviously) and that there was an error on the server side (type 500)

The 5xx range of HTTP response codes are for server side errors, basically there was an error, we couldn't do anything and we are closing the connection.