During my evaluation of JSF 2 I mentioned in a mailing list
post that the implementation of the flash scope doesn’t allow to use it in cluster environments.
If you used JSF 2 in a cluster and did use the flash scope then the application behaviour was unpredictable in the case of fail over.
I just came back to review this and other issues and found that the clustering problem is solved. Since version 2.0.3 of Mojarra JSF you can define where the flash scope is saved.
Just make sure to not store it in application scope but to enable the parameter STORE_FLASH_IN_SESSION_SCOPE.
Best Regards / Viele Grüße