Ok, I reproduced it. I have the main UT window opened, then gmail tab, then ASP.NET forums tab. Nothing else opened IE related.

I then have another UT thread opened close to the main UT tab with a thread. If I click to close this tab only and then try to switch back, I got UT not responding and have to close IE. Same thing seems to happen when I try to close a tab opened from another tab (still opened) from gmail.

See if you can reproduce this scenario.