I have a page which uses ajax() to load some JSON like so..

    $.ajax({'url' : 'array-status.json', 'dataType' : 'json', 'data' : args,
	    'success' : updatestatus,
	    'error' : function(jqXHR, textStatus, errorThrown) {
		disable_spinner();
		$("#errortxt").html("Unable to load antenna data: " + jqXHR.status + " (" + jqXHR.statusText + ")");
	    },
	    'cache' : false
	   });
}

If I kill the web server (a flask instance) I find that the success call back is executed. If the backend is alive but returns an error it behaves as I expect (ie the error call back is executed).

The jqXHR looks like..

qXHR
XMLHttpRequest
abort: function () {
onabort: null
onerror: null
onload: null
onloadend: null
onloadstart: null
onprogress: null
onreadystatechange: function () {}
readyState: 4
response: ""
responseText: ""
responseType: ""
responseXML: null
status: 0
statusText: ""
upload: XMLHttpRequestUpload
withCredentials: false
__proto__: XMLHttpRequest

This is using Chrome 22.0.1229.64 beta.