This problem is produced when smoothing a certain type of polymesh topology:
There are always undesirable artifects when smoothing triangles in zbrush, so always avoid them in your mesh topology. You will also notice that areas of the mesh with greater point densities will smooth at different rates : In certain cases when you have only 3 polygons(4 sided) surrounding a single vertex the polygons in the immediate area converge to a single point, this makes for an increased polygon density in that area, different smoothing rate and a slight pinching effect(not as bad as triangle smoothing).
So in summary the things to avoid in your mesh :
1- 3 sided polygons
2- single vertex surrounded by only 3 four sided polygons
I’ve included in this post two ‘Sculpting Balls’ which I use frequently for my models.Unfortunantly it is impossible to create a mesh that does not contain at least two of these problem areas, so, the only other option is to hide the artifacts - for instance next time when you start modelling a head make sure that the ‘pinch-points’ are situated on the side of the head, maybe hidden away within the ear, so that they have a minimal effect on your modelling process.
Kircho