so I get this nasty pinching at the top of the head and I don't know how to get rid of it. I'm not entirely sure when was created. If I were to guess maybe it happened when I snapped those top vertices along the X axis (toward the mirror modifier). I checked and I don't have any doubles.
I can't find anything on the internet, other than removing doubles. I'm wondering if this has something to do with the modifier itself and I don't really understand the way it works. I got that pinching again when I subdivided the line that goes through the center of the head. So maybe I shouldn't mess with that, I really don't know.
I just did and all the faces are connected. I did notice in face select mode these vertices (they look like vertices), but when I try to delete them in vertex mode, they disappear. I'm not able to select them in any way.
I also tried deleting those faces and fill them again, but the result was even worse than this.
Those 'vertices' in Face Select Mode are the center dots of Faces (so-called zero Faces, because they have a surface area of zero). They should not be there! Select them in Face Select Mode (easiest in Wireframe view!):
and X > Delete > Faces.
Another way to select them, is to enable the 3D Print Toolbox Addon and click on 'Check All' and then on the 'Zero Faces':
Deleting those center dots worked! Thanks!
Merge by distance didn't do anything.
I appreciate the help.