I was trying my hand at modeling a (human) head base mesh, and when I added a subdivision surface modifier to see how it looked when smoothed out, I noticed one of the faces had become round. When I CTRL-Z’d to get back to the mirrored, blocky model, I noticed that face in question had four extra vertices, which I didn’t add. The extra vertices aren’t selectable, so I can’t delete them, and I tried un-subdividing the edges, which also doesn’t work.
I circled the area in question with the grease pencil to show what I’m talking about. The four vertices on the edges that create the quadrilateral need to go - but how?