Bug description: When you use AJAX (simplest example would be autocomplete) in forms, and interrupt the AJAX action, perhaps by clicking another button, you get a popup alert error that says "An HTTP Error 0 occurred".
To recreate the bug a simple technique is just to use Drupal.org. The problem occurs across D6/D7 and all browsers, so the fact that you can see it on D.O is just that that's a good place to demonstrate it. Go to http://drupal.org/project/issues/search/drupal. In the "participants" field type in your userid, but then quickly click the submit button. The example is attached below.
What I expect: The form submission should just take the information it has and interrupt the ajax operation. No error should be displayed.
What I get: An error, widely known in Drupal circles as it's been there for years. But I think we should be able to fix up ajax.js so this doesn't happen.