As of 1.2.2, if you attempt to marshal an impl that extends UnicastRemoteObject and the impl has been unexported, an InternalError will be thrown when RemoteObject.writeObject calls UnicastServerRef.getRefClass. InternalError was appropriate before 1.2.2, because the impl-to-stub replacement code made this situation "not possible". However, now that it's possible, it would seem more appropriate to throw a MarshalException instead.