Properties, deployment.javapi.trace.filename and deployment.javapi.log.filename, when have invalid values set against them, they make applets fails to load with following exceptions under different scenarios:
java.io.FileNotFoundException
or
Exception in thread "Thread-2" java.lang.ClassCastException: sun.net.ProgressMonitor
at sun.plugin.util.GrayBoxPainter.beginPainting(Unknown Source)
at sun.plugin.AppletViewer.initApplet(Unknown Source)
at sun.plugin.AppletViewer.appletInit(Unknown Source)
at sun.plugin.viewer.LifeCycleManager.initAppletPanel(Unknown Source)
at sun.plugin.viewer.IExplorerPluginObject$Initer.run(Unknown Source)
Exception in thread "Thread-3" java.lang.ClassCastException: sun.net.ProgressMonitor
at sun.plugin.util.GrayBoxPainter.beginPainting(Unknown Source)
at sun.plugin.AppletViewer.initApplet(Unknown Source)
at sun.plugin.AppletViewer.appletInit(Unknown Source)
at sun.plugin.viewer.LifeCycleManager.initAppletPanel(Unknown Source)
at sun.plugin.viewer.IExplorerPluginObject$Initer.run(Unknown Source)

Steps to reproduce:
1) Create deployment property with following values:
deployment.javapi.trace.filename=C\:\\temp\test_jpi_trace (correct value C\:\\temp\\test_jpi_trace)
deployment.trace=true
deployment.javapi.log.filename=C\:\\temp\test_jpi_log (correct value C\:\\temp\\test_jpi_log)
deployment.version=1.5.0
deployment.log=true

2)
Launch following applet application
http://sqeweb.sfbay.sun.com/deployment2/jitu/plug-bug/applet_communication/static_var/test1/applet_frame.html

It will fail to load with following exception
java.io.FileNotFoundException: C:\temp est_jpi_trace (The filename, directory name, or volume label syntax is incorrect)

3) Cloase the browser and overwrite the deployment properties file with following enteries
deployment.javapi.trace.filename=test_jpi_trace
deployment.trace=true
deployment.javapi.log.filename=test_jpi_log
deployment.version=1.5.0
deployment.log=true

Launch following applet application again
http://sqeweb.sfbay.sun.com/deployment2/jitu/plug-bug/applet_communication/static_var/test1/applet_frame.html
It will throw following excption
Exception in thread "Thread-2" java.lang.ClassCastException: sun.net.ProgressMonitor
at sun.plugin.util.GrayBoxPainter.beginPainting(Unknown Source)
at sun.plugin.AppletViewer.initApplet(Unknown Source)
at sun.plugin.AppletViewer.appletInit(Unknown Source)
at sun.plugin.viewer.LifeCycleManager.initAppletPanel(Unknown Source)
at sun.plugin.viewer.IExplorerPluginObject$Initer.run(Unknown Source)
Exception in thread "Thread-3" java.lang.ClassCastException: sun.net.ProgressMonitor
at sun.plugin.util.GrayBoxPainter.beginPainting(Unknown Source)
at sun.plugin.AppletViewer.initApplet(Unknown Source)
at sun.plugin.AppletViewer.appletInit(Unknown Source)
at sun.plugin.viewer.LifeCycleManager.initAppletPanel(Unknown Source)
at sun.plugin.viewer.IExplorerPluginObject$Initer.run(Unknown Source)