IE popup: This windows is busy

Sometimes when my asp.net web application throws an unhandled
exception, and I click on the Back button of my browser, then reload
or click on some button of my web app that causes postback, the
browser hangs forever. If then I attempt to close this browser
window, I see the following popup. You know why this happens?

http://gnewsgroup.googlepages.com/thiswindowsisbusy

I am using IE 7 on Windows XP SP2, .net 2.0