Wonky mesh: won't INSET cleanly / attached mesh

In this mesh:

If you select the outer open rim at the right (the ‘bottom’), make it a FACE, and then try to INSET it, it gets all squirrelly in places.

  1. Why is that?
  2. How can one prevent it?

Thanks: mesh attached. --There is a shape key on this mesh, if that makes a difference.

ProblematicMesh.blend (939.3 KB)

You’ve got some wonky geometry around there in those places:

Those verts are almost overlapping, so when you inset, the new edges intersect almost immediately.

Damn, I did multiple Merge By Distances, but this was an imported OBJ and it might be tiny (still haven’t internalized how to immediately determine grid size, my bad (maybe)).

FOR THE NOOBS (like me): The OTHER thing I did is do a Box Select around the problematic areas and check the vertex count – did that like 3 times. My error was that, at the angles I was selecting from AND in a non-wireframe view, one of the vertices was always hidden, so it always seemed like the vertex count was 1. Had I rotated the view more, or gone into a wireframe mode, I would have selected two vertices and solved the issue.

Thanks, Stan.