Java plugin throws an exception if you have a jar file under lib/applet directory. The stack trace is as follows
java.lang.ClassCastException: java.io.File
    at sun.plugin.AppletViewer.loadJarFiles(Unknown Source)
    at sun.applet.AppletPanel.runLoader(Unknown Source)
    at sun.applet.AppletPanel.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)