Public objects/variables defined inside an applet are not accessible from javascript once applet is dragged out of the browser window right after it's loaded. Once JS > Java call is initiated or completed then applet applet is dragged out , then JS > Java call on that object/varibale never fails.
Issue is only seen with IE6/IE7 but not reproducible with FF3.

Steps to reproduce:
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1) Load following applet
http://sqeweb.sfbay.sun.com/deployment2/jitu/plug-bug/ojiliveconnect/src/JSObjectCallTest.html
2) Drag the applet out of browser. Press "Call Java" inside an html page. If error is thrown then bug is reproduced
3) Close the dragged applet. Make sure that applet is getting diplayed inside the browser window.
4) Press "Call Java" inside an html page again. This time error is not thrown but JS is still not able to set the variable value. It will still get displayed as "null"
5) Reload applet. Press "Call Java" inside an html page again. This time one can notice that value of variable is getting set properly to "Value set from JS"
6) Press "Run Test" on applet. Make sure that Java > JS is working fine.
7) Now drag the applet out of the browser window. Press "Call Java" inside an html page again. This time one can notice no error is getting thrown and value of variable is getting set properly to "Value set from JS"

All such scenarios are failing due to this reason on windows.