  The black hole final state proposal reconciles the infalling vacuum with the
unitarity of the Hawking radiation, but only for some experiments. We study
experiments that first verify the exterior, then the interior purification of
the same Hawking particle. (This is the same protocol that renders the firewall
paradox operationally meaningful in standard quantum mechanics.) We show that
the decoherence functional fails to be diagonal, even upon inclusion of
external "pointer" systems. Hence, probabilities for outcomes of these
measurements are not defined. We conclude that the final state proposal does
not offer a consistent alternative to the firewall hypothesis.
