  It is shown that attempts to accurately deduce the magnetic penetration depth
of overdoped BaFe_{1.82}Co_{0.18}As2 single crystals by transverse-field muon
spin rotation (TF-muSR) are thwarted by field-induced magnetic order and strong
vortex-lattice disorder. We explain how substantial deviations from the
magnetic field distribution of a nearly perfect vortex lattice by one or both
of these factors is also significant for other iron-based superconductors, and
this introduces considerable uncertainty in the values of the magnetic
penetration depth obtained by TF-muSR.
