  We study the boundary critical behavior of the three-dimensional Heisenberg
universality class, in the presence of a bidimensional surface. By means of
high-precision Monte Carlo simulations of an improved lattice model, where
leading bulk scaling corrections are suppressed, we prove the existence of a
special phase transition, with unusual exponents, and of an extraordinary phase
with logarithmically decaying correlations. These findings contrast with
na\"ive arguments on the bulk-surface phase diagram, and allow us to explain
some recent puzzling results on the boundary critical behavior of quantum spin
models.
