STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. open the following url with java console open:
http://sqeweb/jsn/users/fred/deployment_ws/reappear/deployment_6_UR_int_ws/deployment/src/plugin/FunctionalTest/FunctionalTest/html/SetFrameApplet.html
2. refresh the page to reload applet
3. see the result in page and output in java console.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
after refreshing, four frames are added on the fly and appear in the page. No exception is printed in java console.
ACTUAL -
after refreshing, no frame appears in the page, and the following exception is printed in java console:
ava.lang.ClassCastException: sun.plugin.dom.html.HTMLBodyElement cannot be cast to org.w3c.dom.html.HTMLFrameSetElement

if I reload the applet by closing the browser, restarting the browser, visiting the url, rather than just refreshing the page, it will work fine.

REPRODUCIBILITY :
This bug can be reproduced always.
The same function implemented by javascript can be found on:
http://sqeweb/jsn/users/fred/deployment_ws/reappear/deployment_6_UR_int_ws/deployment/src/plugin/FunctionalTest/FunctionalTest/html/SetFrameScript.html

it can work when reloading the applet just by refreshing the page