We study the quark-gluon vertex in the limit of vanishing gluon momentum using lattice QCD with two 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 λ1, multiplying the tree-level vertex structure, and the scalar, chiral symmetry breaking form factor λ3 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.