  We study the quark-gluon vertex in the limit of vanishing gluon momentum
using lattice QCD with 2 flavors of O(a) improved Wilson fermions, for several
lattice spacings and quark masses. We find that all three form factors in this
kinematics have a significant infrared strength, and that both the leading form
factor $\lambda_1$, multiplying the tree-level vertex structure, and the
scalar, chiral symmetry breaking form factor $\lambda_3$ are significantly
enhanced in the infrared compared to the quenched (Nf=0) case. These
enhancements are orders of magnitude larger than predicted by one-loop
perturbation theory. We find only a weak dependence on the lattice spacing and
quark mass.
