One of the most fundamental principles of creating a three-dimensional scene is to establish the direction of light. In a complex composition, there may be multiple light sources, and some objects may be obscured by one another, but today I am going to present a much simpler case.
Consider this sign:
The letters themselves are flat, but the two colors are used to create an illusion of volume. If you look at the first couple letters, everything looks pretty good. I can believe that the H and E actually have volume.
However, if you continue looking down the line, you will notice that some letters seem to have completely different sources of light. Here is the same sign with the (probable) direction of light included:
All of a sudden, the light is coming from bottom left at the letter N, even though it was coming from top left for H and E.
Now take a look at the last N. It is a reversed version of the first one, which would imply the light is coming from top right instead.
R is even more interesting. It has so many highlights it is nearly impossible to tell where they are coming from. The bottom of the stem is lit up, which would indicate there is a source below the letter. The left part of it is also lit up, which means there is another one somewhere around top left. In addition, the right half of the leg has the highlight, which would mean there is a source on the right.
This sounds confusing, but not any more confusing than it looks. The letter R looks unnatural because of this problem with the light direction.
Finally, take a look at the D and O. They both have these strange joints where the direction of highlight is supposed to change. If they were actually circular and made out of metal, there would never be a hard line like that. Instead, there would be a soft gradient at the point where the highlight crosses from the outside to the inside.
Looking at this sign makes me wonder — why did somebody design it like this? Why did they try so hard to make some letters look like they are cut out of metal, but bailed out completely on the D and the O? Why did they decide to reverse the light direction arbitrarily?