  Motivated by the recent strong experimental evidence of large \nu_mu-\nu_tau
neutrino mixing, we explore current bounds on the analogous mixing in the
charged lepton sector. We present a general formalism for dimension-6 fermionic
effective operators involving tau-mu mixing with typical Lorentz structure
(\bar{mu} \Gamma tau)(\bar{q}^a \Gamma {q}^b), and discuss their relationship
to the standard model gauge symmetry and the underlying flavor dynamics. We
derive the low-energy constraints on the new physics scale associated with each
operator, mostly from current experimental bounds on rare decay processes of
tau, hadrons or heavy quarks. For operators involving at least one light quark
(u,d,s), these constraints typically give a bound on the new physics scale of a
few TeV or higher. Those operators with two heavy quarks turn out to be more
weakly constrained at the present, giving bounds of a few hundred GeV. A few
scalar and pseudo-scalar operators are free from all current experimental
constraints.
