The near-horizon field B of an old black hole is maximally entangled with the early Hawking radiation R, by unitarity of the S-matrix. But B must be maximally entangled with the black hole interior A, by the equivalence principle. Causal patch complementarity fails to reconcile these conflicting requirements. The system B can be probed by a freely falling observer while there is still time to turn around and remain outside the black hole. Therefore, the entangled state of the BR system is dictated by unitarity even in the infalling patch. If, by monogamy of entanglement, B is not entangled with A, the horizon is replaced by a singularity or "firewall". To illustrate the radical nature of the ideas that are needed, I briefly discuss two approaches for avoiding a firewall: the identification of A with a subsystem of R; and a combination of patch complementarity with the Horowitz-Maldacena final-state proposal.

14 pages, 1 figure. v2: Completely rewritten