  We critically discuss the common belief that the thermal Casimir-Polder force
can always be represented as a Matsubara sum, as it is true for the Casimir
force. On the contrary, an exact treatment of the atom-field coupling reveals
that for a ground-state atom, terms associated with virtual-photon absorption
contribute to the force which are identified as a signature of non-equilibrium
dynamics. Even the equilibrium force on a thermalized atom or molecule may be
overestimated when using the ground-state polarizability instead of its thermal
counterpart.
