This artifact occurs when the scanner moves further than it should vertically, leading to a very "sharp" edge on the features in the image.
You can spot it in the topography profile as "bumps" on the edges of tall features. It's due to hysteresis in piezoelectric scanners.

The AFM image of a test pattern appears to have no artifacts. B: However, a line profile of the test pattern shows overshoot at the top of each of the lines.

How to avoid it.

If you scan more slowly, you will avoid this artifact. However, it is usually only noticable on samples that are very flat, with square features.