I experienced the same thing today. I use MyEclipse for deploying applications and the default behaviour of the web application module is to include all referenced JARs. So even if you don't have jboss-seam.jar copied to your WEB-INF/lib MyEclipse will copy it on deployment if your projects references it. The consequence is, that you end up having two jboss-seam jars - one as usual in the EAR, the other one in your web project.
And now guess what: It turned out that this was exactly the cause for the page flow exception. :-o
So if you use MyEclipse, disable the option to include referenced jars in your web project. If you don't use it, just make sure that jboss-seam jar is where i belongs; in the root of the ear and only there...
Hmm... I use Netbeans and keep jboss-seam.jar where the doctor said :-) in the root of the ear.
I guess, it is incidental superfluous restriction. In this case it can be considered as a bug.
But it may have a deep reason.
It's interesting to see a father's comments.