The bottom will do that when you turn your model into a Sub-d only having a half a head at first. This is because when your in poly mode and want to turn it into a sub-d the last few vertices on the border edge are just loose and move inward when you change it to a sub-d surface. It should smooth out once you attach both sides of the model when your in poly mode then covert it to sub-d again.

It looks like you can still delete a face that is directly under your head, will open up a whole for the neck.


Hope that helps!!!!!!!

Kurt