  General-relativistic stable spacetimes can be made unstable under the
presence of certain nonminimally coupled free scalar fields. In this paper, we
analyze the evolution of linear scalar-field perturbations in spherically
symmetric spacetimes and compare the classical stability analysis with a
recently discussed quantum field one. In particular, it is shown that vacuum
fluctuations lead to natural seeds for the unstable phase, whereas in the
classical framework the presence of such seeds in the initial conditions must
be assumed.
