This bug occurs in FireFox 1.0.4 plus Java 1.5.0_05. It also happens on Mac OS X using Java 1.5.0 Update 3 DP 2 -- it's a bug in GrayBoxPainter.

1. Make sure the Java console is turned on.
2. Go to http://webphysics.davidson.edu/WebTalks/AAPT_Den_Wkshp/moja.html
3. This web site has a badly formed applet (class not found.) That error is correctly reported, but you'll also get this backtrace:

Exception in thread "Thread-4" java.lang.NullPointerException
at sun.plugin.util.GrayBoxPainter.showLoadingError(Unknown Source)
at sun.plugin.AppletViewer.showAppletException(Unknown Source)
at sun.applet.AppletPanel.runLoader(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
java.lang.NullPointerException
at sun.plugin.util.GrayBoxPainter.showLoadingError(Unknown Source)
at sun.plugin.AppletViewer.showAppletStatus(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Exception in thread "thread applet-colors.class" java.lang.NullPointerException
at sun.plugin.util.GrayBoxPainter.showLoadingError(Unknown Source)
at sun.plugin.AppletViewer.showAppletException(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)