Exposed filter AJAX error
When you give the focus to your field, it triggers an ajax call to the server. If you reload the page right away, it breaks the ajax call and throw this error in an alert.

This only happens as you reload the page during an ajax call. As far as i know you can't do anything about it, unless you modify the core I guess - which is "unclean".

You could use the classes added in css to display a loader while there is an ajax call to the server, and let the user know the page is working on something.

EDIT:

Thanks to clive's comment I actually got rid of this alert by overriding Drupal's Drupal.ajax.prototype.error function.

In a js file, you can add :

Drupal.ajax.prototype.error = function (response, uri) {
  // Remove the progress element.
  if (this.progress.element) {
    $(this.progress.element).remove();
  }
  if (this.progress.object) {
    this.progress.object.stopMonitoring();
  }
  // Undo hide.
  $(this.wrapper).show();
  // Re-enable the element.
  $(this.element).removeClass('progress-disabled').removeAttr('disabled');
  // Reattach behaviors, if they were detached in beforeSerialize().
  if (this.form) {
    var settings = response.settings || this.settings || Drupal.settings;
    Drupal.attachBehaviors(this.form, settings);
  }
};


This is a copy of the original error handler callback without the alert() function. This will remove your message, but will also hide any error message returned by Drupal's Ajax.