I have pretty standard mesh with a sub surface modifier (not applied).
Editing has been fine until now - now when I subdivide an edge (one cut) it creates a small black diamond around the new vertex. When I toggle 'Display in viewport' in the subsurf modifier the diamond disappears correspondingly so the two are related. I also can't recreate this in a fresh scene with just the default cube.
Blender version 2.82.7 - any ideas?