  For a certain region of the parameter space $\{M,e,\Lambda\}$, the Cauchy
horizon of a (charged) black hole residing in de Sitter space is classically
stable to gravitational perturbations. This implies that, when left to its own
devices, classical theory is unable to retain full predictive power: the
evolution of physical fields beyond the Cauchy horizon is not uniquely
determined by the initial conditions. In this paper we argue that the Cauchy
horizon of a Reissner-Nordstr\"om-de Sitter black hole must always be unstable
quantum mechanically.
