  The three-particle $N\pi\pi$-state contribution to the QCD two-point function
of standard nucleon interpolating fields is computed to leading order in chiral
perturbation theory. Using the experimental values for two low-energy
coefficients the impact of this contribution on lattice QCD calculations of the
nucleon mass is estimated. The impact is found to be at the per mille level at
most and negligible in practice.
