  We study the two-dimensional, disordered, attractive Hubbard model by the
projector quantum Monte Carlo method and Bogoliubov - de Gennes mean-field
theory. Our results for the ground state show the appearance of a new phase
with charge localization in the metallic regime of the non-interacting model.
Contrary to the common lore, we demonstrate that mean-field theory fails to
predict this phase and is unable to describe the correct physical picture in
this regime.
