Blender.org

http://tinypic.com/r/28uqw5v/5

Why does my head mesh split in half once i add a subdivision

What I see in your screenshot is consistent (in my experience) with having added a mirror modifier, in which you failed to turn on the "merge" option, or alternatively, that you had the merge option on, but with too low a merge limit.

Easiest way to resolve this where you are not, I think, is to temporarily deactivate , or remove the subdivision surface nodifier, select the row(s) of vertices on the midline, and remove doubles. Then reactivate, or reapply the subdivision surface modifier.

ns

I ended up losing that head after turning the wrong plug off *doh

But I looked for duplicates and couldnt find any. But "merge", never heard of it if it happens again I will try it.

Thanks dude