  For an effective field theory in the background of an evaporating black hole
with spherical symmetry, we consider non-renormalizable interactions and their
relevance to physical effects. The background geometry is determined by the
semi-classical Einstein equation for an uneventful horizon where the vacuum
energy-momentum tensor is small for freely falling observers. Surprisingly,
after Hawking radiation appears, the transition amplitude from the Unruh vacuum
to certain multi-particle states grows exponentially with time for a class of
higher-derivative operators after the collapsing matter enters the near-horizon
region, despite the absence of large curvature invariants. Within the
scrambling time, the uneventful horizon transitions towards a firewall, and
eventually the effective field theory breaks down.
