  The definition of matter states on spacelike hypersurfaces of a 1+1
dimensional black hole spacetime is considered. The effect of small quantum
fluctuations of the mass of the black hole due to the quantum nature of the
infalling matter is taken into account. It is then shown that the usual
approximation of treating the gravitational field as a classical background on
which matter is quantized, breaks down near the black hole horizon.
Specifically, on any hypersurface that captures both infalling matter near the
horizon and Hawking radiation, quantum fluctuations in the background geometry
become important, and a semiclassical calculation is inconsistent. An estimate
of the size of correlations between the matter and gravity states shows that
they are so strong that a fluctuation in the black hole mass of order
exp[-M/M_{Planck}] produces a macroscopic change in the matter state.
