The issue can be described as following case.

          var req = new XMLHttpRequest();
          req.open('get', 'file:///test', true);
          req.send();
        

the demo is in this plunkerplunker demo

When the open url is invalid or cors disallowed, the "More tasks executed then were scheduled" will be thrown, it may related to issue #287, and the reason is
https://github.com/angular/zone.js/blob/master/lib/browser/browser.ts#L77,
when request to a invalid url ,the browser will cancel the request and call https://github.com/angular/zone.js/blob/master/lib/browser/browser.ts#L69 immediately, it will become a sync call, so the updateTaskcount (macroTask +1) will not be called.