Currently, if there is an ongoing ajax call and the shadowbox is closed the user will get a browser alert as shown by the included image. The suggested fix for this is to include the following Drupal patch:
https://www.drupal.org/files/issues/drupal_alerts_an_ajax-1232416-295.patch
as suggested on
https://www.drupal.org/node/1232416
And then set the administrative setting to avoid showing the error to the end user