FULL PRODUCT VERSION :
1.6.0_06

ADDITIONAL OS VERSION INFORMATION :
Windows XP

EXTRA RELEVANT SYSTEM CONFIGURATION :
Launching an application using Java Web Start version 1.6.0_06. It works fine without using Java Web Start ie launching from the command-line or IDE.
It also works fine on v1.6.0_05.

A DESCRIPTION OF THE PROBLEM :
Any application using the following jars:

jai_codec-1.1.2_01.jar
jai_core-1.1.2_01.jar
jai_imageio-1.0_01.jar

cannot be launched from Java Web Start v1.6.0_06. The same application can be launched successfully from JWS 1.6.0_05 or command-line v1.6.0_06. Using update 6 it JWS will always through a SecurityException: sealing violation: can't seal package com.sun.media.imageioimpl.stream: already loaded.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Write an jnlp application which uses the following jars:

jai_codec-1.1.2_01.jar
jai_core-1.1.2_01.jar
jai_imageio-1.0_01.jar



EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Application to work as it did on v1.6.0_05.
ACTUAL -
Caused by: java.lang.SecurityException: sealing violation: can't seal package com.sun.media.imageioimpl.stream: already loaded


ERROR MESSAGES/STACK TRACES THAT OCCUR :
Caused by: java.lang.SecurityException: sealing violation: can't seal package com.sun.media.imageioimpl.stream: already loaded
	at java.net.URLClassLoader.defineClass(Unknown Source)
	at java.net.URLClassLoader.access$000(Unknown Source)
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(Unknown Source)
	at com.sun.jnlp.JNLPClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClassInternal(Unknown Source)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Unknown Source)
	... 31 more


REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
I changed the manifest.mf file for each of the jai_*.jar files to remove the line:

sealed: true

Release Regression From : 6u5
The above release value was the last known release where this 
bug was not reproducible. Since then there has been a regression.