  Stellar spectropolarimetry has become extremely popular during the last
decade, and has led to major advances in our understanding of stellar magnetic
fields and of their impact on stellar structure and evolution. Many important
discoveries have been obtained thanks to observations performed with the FORS
low-resolution spectropolarimeters of the ESO Very Large Telescope. We first
review and summarise the major results of a homogeneous re-reduction and
analysis of all single-slit FORS1 spectropolarimetric observations. This work
revealed a non-negligible dependence of the results upon the adopted reduction
and analysis procedure, as well as the presence of instabilities, revealing
that photon noise is not the only source of uncertainty. As a consequence of
our new analysis and assessment of the uncertainties, we are not able to
confirm a large number of magnetic field detections presented in the past for a
variety of stars. We further summarise the results of FORS2 spectropolarimetric
observations of the A0 supergiant HD92207 which allowed us to explore further
the nature of the instabilities, roughly constraining their maximum impact on
the derived Stokes profiles and magnetic field values. We finally present new
results obtained with a further independent pipeline on the FORS2 data of
HD92207, confirming our previous analysis, and discuss simple quality-check
controls which can be performed on the data in order to distinguish between
genuine and spurious signals. All together, our results reveal that the FORS
spectropolarimeters are indeed reliable instruments, when their capabilities
are not pushed beyond the limits of a Cassegrain mounted low-resolution
spectrograph.
