Hello!

I've been using a mod that changes the lighting in CAS, and because of this I've noticed a ‘spot’ or ‘shadow’ appearing on the neck seam of adult sims on one side (male and female, and easier to spot on the mirror). At first I thought it was the fault of the skin or the face overlay, so I tried with vanilla game (except for the lighting mod), but I kept getting that spot or shadow. I tried removing the lighting mod in CAS, but actually, I still see a darker mark on the neck seam of those sims. It's very difficult (you will need to zoom the image) to see it in the screenshots I take without the lighting mod (what the mod does is to emphasize it), but it's a little noticeable. My question is, if it were a defect in the skin, shouldn't it disappear when I put a default replacement of the skin?

If it's noticeable with no mods at all, it's possible there's a (subtly) flawed vertex normal on EA's mesh. It's also quite a sharp polygon, I can see that it's imperfect even in Blender.
CAS lighting is rather harsh with lighting related to normals and normal maps in a way that doesn't happen anywhere else in the game, so I'm not surprised that making it more intense would reveal little imperfections in the 3D model. Bin folder mods like the shadow extender, if you have them, could also reveal such flaws without being the cause itself.

This can also be exacerbated by the individual graphics card and settings related to it- the pixelated shine on the hair in that screenshot is another symptom of this, the correct anti-aliasing settings will make that go away.
But it's also not abnormal for a flaw that minor to exist, the occasional sharp corner catching the light on a mesh that only has 6k triangles to work with just goes with the territory.

Thank you very much for your insight and your point of view. I find it extremely interesting and I learn a lot from everything you tell me in any consultation I post. Thank you very much. Just out of curiosity in case anyone is interested, the CAS lighting mod I was using is the one developed by criisolate, called Pure CAS Lighting Mod for TS3.