  We study the finite supersymmetric loop corrections to fermion masses and
mixing matrices in the generic MSSM. In this context the effects of
non-decoupling chirally-enhanced self-energies are studied beyond leading order
in perturbation theory. These NLO corrections are not only necessary for the
renormalization of the CKM matrix to be unitary, they are also numerically
important for the light fermion masses. Focusing on the tri-linear A-terms with
generic flavor-structure we derive very strong bounds on the chirality-changing
mass insertions delta^{f\,LR,RL}_{IJ} by applying 't Hooft's naturalness
criterion. In particular, the NLO corrections to the up quark mass allow us to
constrain the unbounded element delta^{u\,RL}_{13} if at the same time
$\delta^{u\,LR}_{13}$ is unequal to zero. Our result is important for
single-top production at the LHC.
