  During the last decade, the FORS1 instrument of the ESO VLT has been
extensively used to study stellar magnetism. A number of interesting
discoveries of magnetic fields in several classes of stars have been announced,
many of which obtained at a ~3 sigma level; some of the discoveries are
confirmed by measurements obtained with other instruments, some are not. Here
we investigate the reasons for the discrepancies between the results obtained
with FORS1 and those obtained with other instruments.
  Using the ESO FORS pipeline, we have developed a semi-automatic procedure for
magnetic field determination. We have applied this procedure to the full
content of circular spectropolarimetric measurements of the FORS1 archive. We
have devised and applied a number of consistency checks to our field
determinations, and we have compared our results to those previously published
in the literature.
  We find that for high signal-to-noise ratio measurements, photon noise does
not account for the full error bars. We discuss how field measurements depend
on the specific algorithm adopted for data reduction, and we show that very
small instrument flexures, negligible in most of the instrument applications,
may be responsible for some spurious field detections in the null profiles.
Finally, we find that we are unable to reproduce some results previously
published in the literature. Consequently, we do not confirm some important
discoveries of magnetic fields obtained with FORS1 and reported in previous
publications.
  Our revised field measurements show that there is no contradiction between
the results obtained with the low-resolution spectropolarimeter FORS1 and those
obtained with high-resolution spectropolarimeters. FORS1 is an instrument
capable of performing reliable magnetic field measurements, provided that the
various source of uncertainties are properly taken into account.
