  In a paper posted on the arXiv a few weeks ago Berti, Brito and Cardoso
\cite{Berti+14} suggest that ultra-high-energy particles can emerge from
collisions in a black hole's ergosphere. This can happen if the process
involves a particle on an outgoing trajectory very close to the black hole.
Clearly such a particle cannot emerge from the black hole. It is argued
\cite{Berti+14} that this particle can arise in another collision. Thus the
process involves two collisions: one in which an outgoing particle is produced
extremely close to the horizon, and a second one in which energy is gained. The
real efficiency of this process should take into account, therefore, the energy
needed to produce the first particle. We show here that while this process is
kinematically possible, it requires a deposition of energy that is divergently
large compared with the energy of the escaping particle. Thus, in contradiction
to claims of infinitely high efficiencies, the efficiency of the combined
process is in fact extremely small, approaching zero for very high output
energies. Even under more general conditions than those considered in
\cite{Berti+14} the total energy gain never diverges, and is larger only by a
factor of a few than the energy gain of the original collisional Penrose
process that takes place between two infalling particles
\cite{Piran+75,PiranShaham77,Bejger+12}
