Could be any number of things
Check that you have "Keep faces together" toggled on, under the Mesh menu
Under the Normals menu, click "Conform"
Check for unmerged verts
Doesn't look like a double edge going on, but in other cases you'd want to check for that too
EDIT: I can't even recreate that... Sorry I can't help more
Last edited by Acid44; 02-01-2013 at 06:09 AM.