While working on some simple models today I encountered a strange visual bug/glitch when using the Gumball on an edge. This happened on the latest build of Rhino 8.
When moving one edge over an existing one, it left a strange surface behind as seen in the screenshots. It can’t be removed as it is seen as part of the closed polysurface.
I can repeat this, if you have CheckNewObjects on, it will put up a message that a bad object has been created. Rhino doesn’t know yet how to actually remove the surface that you “eliminated” by moving its edge to zero, so it leaves it there as an invalid object (a surface that has zero edge length on two sides and zero area). The phantom surface you see is just a display mesh artifact, which is why you can’t select it.
Direct editing of objects in Rhino is rather limited. Making this more robust will largely require a case-by-case effort or a substantial re-write of Rhino.
I’ve put this specific case on the list as RH-78708 MoveEdge: Collapse planar face creates bad object
-wim