Reliability problems have been seen that appear to be related to re-launches that occur for applets launched via JNLP. 6723194 describes some of the scenarios in which the problems have been seen. Due to the need to minimize changes in 6u10 at this point in the release cycle, the fix for 6723194 will only cease throwing InternalErrors, but not actually diagnose the root cause of why those InternalErrors were raised. It is critically important to have high reliability and to understand why intermittent problems are being seen in stress situations such as launching multiple applets on the same web page that all use JNLP and which cause relaunches.
The Scene Graph demos at http://scenegraph-demos.dev.java.net/demo-applets.html , linked from https://jdk6.dev.java.net/plugin2/#DEMOS , have been seen to demonstrate the problem.
Investigation during the fix of 6723194 indicates that the root cause of this problem might be duplicate relaunch messages being sent for a particular applet instance. More investigation is needed.