"wings" appear when I add a subsurface modifier to some simple geometry I've got some very simple geometry (a cube with some extruded subdivisions). When I add a subsurf modifier to it these &quot;wings&quot; appear. They seem to be edges that connect vertices that weren't originally connected by edges. However, I can't select and/or delete them in any way.
How do I get rid of these?
I've checked the normals and they seem to be in order in the area that the wings appear in. Blender can't find any non-manifold geometry in the source.
Blend file: https://drive.google.com/file/d/1xLYRTWBchK3zBOWLDAxSLF-MlWVcGdpy/view?usp=sharing
Ngons (faces with more than 4 sides)

Join some of those vertices (manually subdivide edges where needed) so that you have at most 4 sides for each face.
You have a lot going on in this scene. Blender can highlight ngons for you.
In Edit mode, go to Select -&gt; Select All By Trait -&gt; Faces By Sides.
A little menu will appear in the lower-left. Choose 4 and Greater Than.
Do this with the Subdivision Surface modifier turned off, it will be easier.
Here is a .gif showing you how to isolate islands to work on (subdivision surface modifier has been turned off)