  Studies of deep photometry of galaxies have presented discoveries of excess
light in surface-brightness and colour profiles at large radii in the form of
diffuse faint haloes and thick discs. In a majority of the cases, it has seemed
necessary to use exotic stellar populations or alternative physical solutions
to explain the excess. Few studies have carefully scrutinized the role of
scattered light in this context. I explore the influence of scattered light on
ground-based observations of haloes and thick discs around edge-on galaxies,
haloes around face-on disc galaxies, host galaxies around blue compact galaxies
(BCGs), and haloes around elliptical galaxies. Surface-brightness structures of
all considered types of galaxies are modelled and analysed to compare
scattered-light haloes and thick discs with measurements. I simulate the
influence of scattered light and accurate sky subtraction on simplified
S\'ersic-type and face-on disc galaxy models. All galaxy models are convolved
with both lower-limit and brighter point spread functions (PSFs); for a few
galaxies it was possible to use dedicated PSFs. The results show bright
scattered-light haloes and high amounts of red excess at large radii and faint
surface brightnesses for nearly all types of galaxies; exceptions are the
largest elliptical-type galaxies where the influence of scattered light is
smaller. Studies have underestimated the role of scattered light to explain
their surface-brightness profiles. My analysis shows surface-brightness
profiles that include scattered light that are very similar to and overlap
measurements at all radii. The derivation of physical properties of haloes,
thick discs, and BCG hosts from diffuse data is misleading since accurate and
radially extended PSFs are non-existent. Significantly improved analyses that
include new measurements of PSFs are required to study diffuse haloes further.
