With latest 6u18 nightly bundels sometimes browser/VM hangs while running the mixed certification scenario, where different classes signed using different signer CAs exits under differnt packages.
Stack trace when VM/browser hanged (NOt able to click on the second security pop-up dialog):
http://sqeweb.sfbay.sun.com/deployment2/jitu/deployment_mustang_exec_ws/deployment/src/plugin/RSATestsuite/RSATestsuite/mystack.txt

Steps to reproduce:
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1) Try to load any of the following applets
http://sqeweb.sfbay.sun.com/deployment2/jitu/deployment_mustang_exec_ws/deployment/src/plugin/RSATestsuite/RSATestsuite/html/MixedCertDifferentPackage/MixedCerificationSigned4.html
http://sqeweb.sfbay.sun.com/deployment2/jitu/deployment_mustang_exec_ws/deployment/src/plugin/RSATestsuite/RSATestsuite/html/MixedCertDifferentPackageJNLP/JNLPMixedCertTestDraw.html

2) There is going to be 2 security pop-ups coming from different signer CAs. If at any point of time browser hangs and you are not able to click any button on the security dialog then bug is reporduced