[jQuery] Why do both ajaxError and ajaxSuccess fire if the request fails or is a success?

[devioustree]

I was wondering why this code was giving me the error _and_ parsing my
XML:

        function getXML () {
                $.ajax({
                        dataType: "xml",
                        error: errorMessage("The data could not be loaded."),
                        success: function (msg){parseXML(msg);},
                        timeout: 10000,
                        url: xmlUrl
                });
        };

when I read (from John himself I believe) that both error and success
are fired if the request fails or is successful. Can somebody explain
the logic here? Have I just misunderstood something?

Also, is it possible to have the error message show only when the
request fails and success show only when the request works?

Cheers.