  Multi-particle states with additional pions are expected to be a
non-negligible source of excited-state contamination in lattice simulations at
the physical point. It is shown that baryon chiral perturbation theory can be
employed to calculate the contamination due to two-particle nucleon-pion states
in various nucleon observables. Leading order results are presented for the
nucleon axial, tensor and scalar charge and three Mellin moments of parton
distribution functions (quark momentum fraction, helicity and transversity
moment). Taking into account phenomenological results for the charges and
moments the impact of the nucleon-pion-states on lattice estimates for these
observables can be estimated. The nucleon-pion-state contribution results in an
overestimation of all charges and moments obtained with the plateau method. The
overestimation is at the 5-10% level for source-sink separations of about 2 fm.
The source-sink separations accessible in contemporary lattice simulations are
found to be too small for chiral perturbation theory to be directly applicable.
