Without a screenshot its hard to say, but it might be what is called an LOD (Level of Detail) artifact which can result if your machine isn't beefy enough to handle the graphics load you are throwing at it. Most of the game meshes come with 2 variants: a low-res and a hi-res model. The low-res model is typically only rendered when objects are in the distance (where you can't notice) or if the graphics card is under excessive load.
If its just one sim, it could be corruption of the face data in the character file. Usually, using the mirror's Change Appearance function (and then saving) will rewrite that part of the character file correcting any errors that are in it.
Anyway, a couple of things to pursue, although I can't say I've heard of this happening with just the nose
