  Naive intuition about scale decoupling breaks down in the presence of fermion
masses. Kinematic enhancements can greatly extend the range where one needs to
keep a finite mass in calculations to obtain a correct result at even the O(1)
level. Treating a light fermion as massive though, leads to a known but
somewhat obscure paradox, a seeming leading-order sensitivity to an arbitrarily
small mass. We show how a proper formulation in effective field theory not only
resolves the physical conundrum, but answers the very practical question of
when fermion masses are required. This has important implications for the
development of shower Monte Carlo above the weak scale.
