Applet using SingleInstance service should prevent multiple versions of the same applets running at the same time inside the same browser session. It's working fine on IE6 but not on IE7 and FF3.
1)Try to laod the following applet: http://nicole1.sfbay.sun.com:8080/plugin_tests/JNLPSupport/applets/AppletBasics/JNLPSingleInstanceService1-jnlp-applet-rel1.html

2) If on IE7 and FF3 you can notice second applet getting started after a while(on top of already running applet) then bug is reproduced.