This issue is reported at java.net forum:

http://forums.java.net/jive/thread.jspa?messageID=333410

Sometimes the following exception is observed in Java Console when applet is being destroyed:

java.lang.IllegalThreadStateException: forbid thread creation in disposed TG
at sun.plugin.security.ActivatorSecurityManager.checkAccess(Unknown Source)
at java.lang.ThreadGroup.checkAccess(Unknown Source)
at java.lang.Thread.init(Unknown Source)
at java.lang.Thread.<init>(Unknown Source)
at java.awt.EventDispatchThread.<init>(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.awt.EventQueue.initDispatchThread(Unknown Source)
at java.awt.EventQueue.postEventPrivate(Unknown Source)
at java.awt.EventQueue.postEvent(Unknown Source)
at java.awt.EventQueue.invokeAndWait(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

It's clear that it's thrown when AWT tries to create a new EventDispatchThread instance for disposed AppContext.