Steps to reproduce:
  1. Make sure Firebug is active. Go to to "Console" tab for this page.
  2. Click the "Do It" button below.
After about ten seconds, there should be three of these errors:
onreadystatechange FAILS Error: Permission denied for <http://dev.pgengler.net> to create wrapper for object of class UnnamedClass
Error: Permission denied for <http://dev.pgengler.net> to create wrapper for object of class UnnamedClass readystatechange" in the console.

What it's doing: When the button is clicked, two XmlHttpRequest() requests are made; the first has a 10-second delay on the server end while the second comes back instantly. The errors come when the first request (the one with the delay) finishes.
If you don't see the problem the first time, refresh the page and try again. This usually happens the first time but if not the second time does it.