By using smooth shading (i.e. interpolation of normals), an object can have a smooth look despite low polygon count. A nasty artifact that can happen when casting shadow rays is the Shadow Line Artifact. Since shadow rays originate from a certain position and don't usually take the normal into account, a complete face on a smooth object will either be shadowed or not, revealing the hard edges again:
The shadow is, so to speak, 'correct' and the weird appearance results from the normal map interpolation 'cheat'. Nevertheless, it is visually distracting. The linked article above, written in 2009, suggests removing shadows or self-shadows on objects that show this kind of artifact, which (especially for non-convex objects) is not a satisfying solution.
This is a very common problem inherent to ray tracing algorithms. I wonder if there is any better solution to it than subdividing the mesh into flat-shaded high-poly or getting rid of the shadows altogether.