Mad, Martian, but not mad Martian pain
Functionalism cannot accommodate the possibility of mad pain—pain whose causes and effects diverge from those of the pain causal role. This is because what it is to be in pain according to functionalism is simply to be in a state that occupies the pain role. And the identity theory cannot accommodate the possibility of Martian pain—pain whose physical realization is foot-cavity inflation rather than C-fibre activation (or whatever physiological state occupies the pain-role in normal humans). After all, what it is to be in pain according to the identity theory is to be in whatever state that occupies the pain role for us.