  Recent N-body simulations have shown that Einasto radial profiles provide the
most accurate description of dark matter halos. Predictions based on the
traditional NFW functional form may fail to describe the structural properties
of cosmic objects at the percent level required by precision cosmology. We
computed the systematic errors expected for weak lensing analyses of clusters
of galaxies if one wrongly models the lens density profile. Even though the NFW
fits of observed tangential shear profiles can be excellent, viral masses and
concentrations of very massive halos (>~ 10^{15}M_Sun/h) can be over- and
underestimated by ~10 per cent, respectively. Misfitting effects also steepen
the observed mass-concentration relation, as observed in multi-wavelength
observations of galaxy groups and clusters. Based on shear analyses, Einasto
and NFW halos can be set apart either with deep observations of exceptionally
massive structures (>~ 2\times10^{15}M_Sun/h) or by stacking the shear profiles
of thousands of group-sized lenses (>~ 10^{14}M_Sun/h).
