Judder. Most displays refresh at a maximum of 60 Hz, so any framerate that doesn't divide evenly into 60 means showing frames an uneven number of times, which leads to a noticeable decrease in smoothness even if the framerate is constant.
Code:
60 FPS
1 2 3 4 5 6
30 FPS
1 1 2 2 3 3
20 FPS
1 1 1 2 2 2
45 FPS
1 2 3 3
4 5 6 6
7 8 9 9
It's basically a consistent, intermittent framerate drop. You can mitigate it with various blending/processing techniques, but you don't want to deal with it if you don't have to.