  We carefully re-examine the conditions of validity for the consistent
derivation of the Lifshitz-Matsubara sum formula for the Casimir pressure
between metallic plane mirrors. We recover the usual expression for the lossy
Drude model, but not for the lossless plasma model. We give an interpretation
of this new result in terms of the modes associated with the Foucault currents
which play a role in the limit of vanishing losses, in contrast to common
expectations.
