java 6up19 introduced

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Ensuring Application and Applet Security when Mixing Signed and Unsigned Code

Signed Java Web Start applications and applets that contain signed and unsigned components could potentially be unsafe unless the mixed code was intended by the application vendor. As of this release, when mixed code is detected in a program, a warning dialog is raised. Mixing Signed and Unsigned Code explains this warning dialog and options that the user, system administrator, developer, and deployer have to manage it.
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more info at http://java.sun.com/javase/6/docs/technotes/guides/jweb/mixed_code.html