Low light ambient shots with lots of dark areas (typical of when
perhaps shooting in theaters, night clubs, or night time street
scenes) will characteristically show "ghosting" of light sources,
the product of reflections when a filter is used on a lens (such
as the typical UV filter).

The image above shows this effect -- the light source in the
picture (lower right) shows a flipped mirrored "ghost" in the
upper left.

Such ghosting issues can sometimes be resolved just by removing
the filter from the lens when shooting under such conditions.

For my own part, I typically have UV filters on all of my lenses
(it protects the front lens element, and I don't feel any concerns
about just breathing on it and then wiping with my T-shirt just
to clean it off).

But for night time photography, such as the situation in the
example above, the UV filter comes off the lens so as to avoid
this ghosting problem.