Consider the following situation:

You have an EAR with the following in a shared ejb-jar.xml lib:

public interface X..
and
@Stateless class MyEJB{ private @Inject X; ...

The EAR also contains 2 web applications which have the following implementations of X:

A implements X in webappA
B implements X in webappB

regarding on the webapp, the proxy either points to an instance of A or B
which are NOT castable to each other.

This is basically only possible if we only inject a subclass (or implementation if X is a pure interface) of X as proxy into the shared EJB.

Not sure though how this could be unit tested via the TCK.