Flavor extrapolations and staggered fermions
  A popular approximation in lattice gauge theory is an extrapolation in the number of fermion species away from the four fold degeneracy natural with the staggered fermions formulation. I show that at finite lattice spacing and for an odd number of flavors this extrapolation misses terms which, on general principles, must be present in the continuum theory. For a correct continuum limit, this forces unphysical singularities in parameter regions where continuum physics is smooth and all physical particles are massive. These singularities are not expected with other lattice regulators. Finally, I argue that unnatural constraints on certain correlation functions appear even when all quarks are massive. 
Michael Creutz