Recently I began working on a 3ds max modelling tutorial. The tutorial explains how to model a head. You have to use a Symmetry modifier to mirror one half of the head.
I did this and everything worked fine..except there's a thin line between the two halfs of the face. The halfs don't really weld together as one. It looks more like the face is cut in to two pieces and is pasted together aftwerwards (sorry I don't really know how else to explain :S). And I don't really know how to fix this.