  We consider generic nonextremal stationary dirty black holes. It is shown
that in the vicinity of any bifurcation surface the energy of collision of two
particles in the centre of mass frame can grow unbound. This is a generic
property that, in particular, includes collisions near the inner black hole
horizon analyzed earlier by different methods. The similar results are also
valid for cosmological horizons. The case of the de Sitter metric is discussed.
