FULL PRODUCT VERSION :
Java Plug-in 1.6.0_18
Using JRE version 1.6.0_18-ea-b02 Java HotSpot(TM) Client VM

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
When using the next generation java plug-in, and a class file cannot be found in the archive jar file(s), findAppletJDKLevel is thrown as follows:

java.lang.NullPointerException
at sun.plugin2.applet.Plugin2Manager.findAppletJDKLevel(Unknown Source)
at sun.plugin2.applet.Plugin2Manager.createApplet(Unknown Source)
at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Exception: java.lang.NullPointerException
Ignored exception: java.lang.NullPointerException



STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1) Close all browsers, ensuring that there are no plugin coffee cup icons in the Windows taskbar.
2) Open the Java Control panel (Start->Control Panel->Java).
2.5) On the General tab, clear cache by clicking Settings..., Delete Files..., OK.
3) Open the Advanced tab.
4) Expand the Java Plug-in tree option.
5) Ensure that the "Enable the next-generation Java Plug-in" option is checked and click OK.
6) Visit page http://www.printdesktop.com/IeTabbedCrash/IeTabbedCrash.html
7) View the java console stacktrace. It should include findAppletJDKLevel message.

Now, to see the correct error message (NoClassDefFoundError) displayed, follow these steps:
1) Close all browsers, ensuring that there are no plugin coffee cup icons in the Windows taskbar.
2) Open the Java Control panel (Start->Control Panel->Java).
2.5) On the General tab, clear cache by clicking Settings..., Delete Files..., OK.
3) Open the Advanced tab.
4) Expand the Java Plug-in tree option.
5) Ensure that the "Enable the next-generation Java Plug-in" option is NOT checked and click OK.
6) Visit page http://www.printdesktop.com/IeTabbedCrash/IeTabbedCrash.html
7) View the java console stacktrace. It should include NoClassDefFoundError error message.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
See a useful stacktrace:

java.lang.NoClassDefFoundError: IeTabbedCrash$2
at java.lang.Class.getDeclaredConstructors0(Native Method)
at java.lang.Class.privateGetDeclaredConstructors(Unknown Source)
at java.lang.Class.getConstructor0(Unknown Source)
at java.lang.Class.newInstance0(Unknown Source)
at java.lang.Class.newInstance(Unknown Source)
at sun.applet.AppletPanel.createApplet(Unknown Source)
at sun.plugin.AppletViewer.createApplet(Unknown Source)
at sun.applet.AppletPanel.runLoader(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: IeTabbedCrash$2
at sun.applet.AppletClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.applet.AppletClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 10 more
Ignored exception: java.lang.NoClassDefFoundError: IeTabbedCrash$2

ACTUAL -
See this misleading stacktrace:

java.lang.NullPointerException
at sun.plugin2.applet.Plugin2Manager.findAppletJDKLevel(Unknown Source)
at sun.plugin2.applet.Plugin2Manager.createApplet(Unknown Source)
at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Exception: java.lang.NullPointerException
Ignored exception: java.lang.NullPointerException

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.lang.NullPointerException
at sun.plugin2.applet.Plugin2Manager.findAppletJDKLevel(Unknown Source)
at sun.plugin2.applet.Plugin2Manager.createApplet(Unknown Source)
at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
http://www.printdesktop.com/IeTabbedCrash/IeTabbedCrash.jar
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Disable the Next-generation java plug-in.