  The faint regions of galaxies, groups and clusters hold important clues about
how these objects formed, and surface photometry at optical and near-infrared
wavelengths represents a powerful tool for studying such structures. Here, we
identify a hitherto unrecognized problem with this technique, related to how
the night sky flux is typically measured and subtracted from astronomical
images. While most of the sky flux comes from regions between the observer and
the target object, a small fraction - the extragalactic background light (EBL)
- comes from behind. We argue that since this part of the sky flux can be
subject to extinction by dust present in the galaxy/group/cluster studied,
standard reduction procedures may lead to a systematic oversubtraction of the
EBL. Even very small amounts of extinction can lead to spurious features in
radial surface surface brightness profiles and colour maps of extended objects.
We assess the likely impact of this effect on a number of topics in
extragalactic astronomy where very deep surface photometry is currently
attempted, including studies of stellar halos, starburst host galaxies, disc
truncations and diffuse intragroup/intracluster light. We argue that EBL
extinction may provide at least a partial explanation for the anomalously red
colours reported for the halos of disc galaxies and the hosts of local
starburst galaxies. EBL extinction effects also mimic truncations in discs with
unusually high dust opacities, but are unlikely to be the cause of such
features in general. Failure to account for EBL extinction can also give rise
to a non-negligible underestimate of intragroup and intracluster light at the
faintest surface brightness levels currently probed. (Abridged)
