Hi wisty, that's just a display glitch it's best to just ignore it. It happens when the display mesh didn't generate quite enough triangles in that area.

If you set the mesh angle to a low value like Pilou writes above that will make a denser display mesh but on larger models it will be extremely dense and bog things down so it's usually best to leave that at its default value of 10 degrees.

When you go to export your model you can get rid of those artifacts by increasing the density in the exported mesh by moving the slider towards "More polygons" and also by adding in a distance limiting value in "Divide larger than".

There are various kinds of display glitches that can happen which are a byproduct of it being tuned to be fast.

- Michael