  We present a detailed discussion of the redshift errors associated to the ESO
Slice Project measurements. For a subsample of 742 galaxies with redshifts
determined both from the absorption lines (V_{abs}) and from the emission lines
(V_{emi}), we find an average difference <V_{abs} - V_{emi}> ~ +100$ km/s. We
find that a similar effect is present in another, deeper redshift survey, the
Durham/Anglo-Australian Telescope faint galaxy redshift survey, while is absent
in surveys at brighter magnitude limits. We have investigated in detail many
possible sources of such a discrepancy, and we can exclude possible zero-point
shifts or calibration problems. We have detected and measured systematic
velocity differences produced by the different templates used in the
cross-correlation. We conclude that such differences can in principle explain
the effect, but in this case the non-trivial implication would be that the
best-fitting template does not necessarily give the best velocity estimate. As
we do not have any a priori reason to select a template different from the
best-fitting one, we did not apply any correction to the ESO Slice Project
velocities. However, as for a small number of galaxies the effect is so large
that it is likely to have a physical explanation, we have also taken into
account the possibility that the discrepancy can be partly real: in this case,
it might help to understand the role of gas outflows in the process of galaxy
evolution. In view of the future large spectroscopic surveys, we stress the
importance of using different templates and making them publicly available, in
order to assess the amplitude of systematic effects, and to allow a direct
comparison of different catalogues.
