  We argue that flat space amplitudes for the process $ 2 \to n$ gravitons at
center of mass energies $\sqrt{s}$ much less than the Planck scale, will
coincide approximately with amplitudes calculated from correlators of a
boundary CFT for AdS space of radius $R\gg L_P$, only when $n < R/L_P$ . For
larger values of $n$ in AdS space, insisting that all the incoming energy
enters "the arena" [arXiv:hep-th/9901079], implies the production of black
holes, whereas there is no black hole production in the flat space amplitudes.
We also argue, from unitarity, that flat space amplitudes for all $n$ are
necessary to reconstruct the complicated singularity at zero momentum in the $2
\to 2$ amplitude, which can therefore not be reproduced as the limit of an AdS
calculation. Applying similar reasoning to amplitudes for real black hole
production in flat space, we argue that unitarity of the flat space S-matrix
cannot be assessed or inferred from properties of CFT correlators.
