that means not all your verts are lined up properly along the 0 axis. Grab all the verts in one go that you want to be on that line and scale them on the axis (probably z) untill they line up (the scaling handle should sit under the middle scale handle).
Then mirror them and then smooth them. Never smooth first always do it last.
Alan
Technical Director - Framestore
Currently working on: Your Highness
IMDB