  We comment on a recent publication [1] by Fosco, Lombardo and Mazzitelli on
Casimir energies for material slabs (`finite width mirrors') and report a
discrepancy between results obtained there for a single mirror and some
previous calculations. We provide a simple consistency check which proves that
the method used in [1] is not reliable when applied to approximations of
piecewise constant profile of the mirror. We also present an alternative method
for calculation of the Casimir energy in such systems based on our recent work.
Our results coincide both with perturbation theory and with some older
\cite{Bordag'95} and more recent \cite{Vassilevitch'08} calculations, but
differ from those of [1].
