Breakdown of the Universality Hypothesis in Directed Abelian Sandpile   Models
  We show that in a broad class of directed abelian sandpile models that had been expected to have the same exponents as the Dhar-Ramaswamy model, the avalanche exponent depends upon the details of the interaction, calling into question the general existence of universality classes in self organized critical models. 
Rick Tully and George Reiter