The variables success, error, dataType, global and timeout are only initialized properly if theres only one argument passed to jQuery.ajax as object.

if ( !url ) {
	ret = type.complete;
	var success = type.success;
	var error = type.error;
	var dataType = type.dataType;
	var global = typeof type.global == "boolean" ? type.global : true;
	var timeout = typeof type.timeout == "number" ? type.timeout : jQuery.timeout;
	var ifModified = type.ifModified || false;
	data = type.data;
	url = type.url;
	type = type.type;
}

Otherwise, for example global is undefined and no ajax events are triggered

if ( global && ! jQuery.active++ )
	jQuery.event.trigger( "ajaxStart" );