When applet using JNLP is launched inside the browser then "Staring Application" dialog appears which goes away by itself most of the times. But sometimes it stays there covering the displayed applet behind it. One need to to click on "Cancel" button to make it dispose away in order to see/interact with the displaying applet.
Run applets from the following page and if you happened to noticed the above mentioned behavior then bug is reproduced
http://nicole1.sfbay.sun.com:8080/plugin_tests/JNLPSupport/index.html