Lower bound on the mass of a black hole
  We consider gravity coupled to a massive field whose Compton's wavelength is far larger than the Planck's length. In the low energy effective action for gravity, thus, it is the perturbation in the Compton's wavelength that breaks first as the sub-sub-leading quantum perturbation grows stronger. When this break occurs, we can not trust the perturbative information about the form of the low energy effective action. We translate this break into the lowest limit on the mass of a classical black hole. In D=4, using the electron's mass, this requires the black hole to be heavier than 10^{14} kg. 
Qasem Exirifard