  The most general action for a scalar field coupled to gravity that leads to
second order field equations for both the metric and the scalar --- Horndeski's
theory --- is considered, with the extra assumption that the scalar satisfies
shift symmetry. We show that in such theories the scalar field is forced to
have a nontrivial configuration in black hole spacetimes, unless one carefully
tunes away a linear coupling with the Gauss--Bonnet invariant. Hence, black
holes for generic theories in this class will have hair. This contradicts a
recent no-hair theorem, which seems to have overlooked the presence of this
coupling.
