It looks more easily to replicate as following:
1.when the pagefile include such as: <script language="JavaScript" src="<%=FO_ROOT %>js/menu/test.js"></script>
no matter what is in test.js, even it's empty, the sessionid may be changed.
2.In IE refresh the page quickly, the session id may changed easily.
[Message sent by forum member 'hanjingdi' (hanjingdi)]
http://forums.java.net/jive/thread.jspa?messageID=244008