  In this paper we argue that a firewall is simply a manifestation of an
inconsistent truncation of non-perturbative effects that unitarize the
semiclassical black hole. Namely, we show that a naive truncation of quantum
corrections to the Hawking spectrum at order ${\cal O}(e^{-S})$, inexorably
leads to a "localised'' divergent energy density near the black hole horizon.
Nevertheless, in the same approximation, a distant observer only sees a
discretised spectrum and concludes that unitarity is achieved by ${\cal
O}(e^{-S})$ effects. This is due to the fact that instead, the correct quantum
corrections to the Hawking spectrum go like ${\cal O}( g^{tt} e^{-S})$.
Therefore, while at a distance far away from the horizon, where $g^{tt}\approx
1$, quantum corrections {\it are} perturbative, they {\it do} diverge close to
the horizon, where $g^{tt}\rightarrow \infty$. Nevertheless, these
"corrections" nicely re-sum so that correlations functions are smooth at the
would-be black hole horizon. Thus, we conclude that the appearance of firewalls
is just a signal of the breaking of the semiclassical approximation at the Page
time, even for large black holes.
