A fellow programmer had Google's popup killer installed in her Internet Explorer browser. We noticed that her BSP behavior was different from mine - when I changed my URL, the underlying BSP (CRM_BSP_FRAME in this case) would terminate and restart, actually creating a new IE window. It's almost invisible to the end-user when this happens. However, the data in the screen is reset.
If a popup blocker is installed, the new window is suppressed, leaving you looking at the old screen. Watch out for it - it took a long while to spot this problem.