  During the last decade, the FORS1 instrument of the ESO Very Large Telescope
has been used to obtain low resolution circular polarized spectra for about 500
stars, with the aim of measuring their mean longitudinal magnetic fields.
Magnetic field estimates were obtained by different authors, using different
software tools. Several interesting detections were obtained at a 3 sigma
level; some of them were eventually confirmed by follow-up investigations, some
of them were not. This raises issues about the reliability of the stated
uncertainties of some of the published field values. To investigate these
problems, we have developed a semi-automatic procedure for magnetic field
determination, which includes self-consistent checks for field detection
reliability. We have applied our procedure to the full content of single star
("fast mode") circular spectropolarimetric measurements of the FORS1 archive,
and explored the details and interagreement of various methods for data
reduction. We have finally produced a catalogue of FORS1 longitudinal field
measurements which includes about 1000 entries. Here we critically review the
previously published FORS1 measurements, and, based on our results, we suggest
that the incidence of the magnetic field in various classes of stars should be
revised.
