  For the well studied halo star HD 122563 and the four stars in the globular
cluster NGC 6397, we determine NLTE abundances of iron using classical
plane-parallel model atmospheres. Each star reveals a discrepancy in abundances
between the Fe I lines arising from the ground state and the other Fe I lines,
in qualitative agreement with the 3D-LTE line formation predictions, however,
the magnitude of the observed effect is a factor of 2 smaller compared with the
predicted one. When ignoring the Fe I low-excitation lines, the NLTE abundances
from the two ionization stages, Fe I and Fe II, are consistent in each
investigated star. For the subgiants in NGC 6397, this is only true when using
the cooler effective temperature scale of Alonso et al. (1999). We also present
full 3D-LTE line formation calculations for some selected iron lines in the
solar and metal-poor 4480/2/-3 models and NLTE calculations with the
corresponding spatial and temporal average <3D> models. The use of the <3D>
models is justified only for particular Fe I lines in particular physical
conditions. Our NLTE calculations reproduce well the centre-to-limb variation
of the solar Fe I 7780 A line, but they are unsuccessful for Fe I 6151 A. The
metal-poor <3D> model was found to be adequite for the strong Fe I 5166 A (Eexc
= 0) line, but inadequite in all other investigated cases.
