When the brain tells the mouth to arrange itself in smile position but forgets to tell the rest of the face (eyes, temples etc.) to do the same. Sometimes revealing the lower teeth, it is often menacing and always insincere.
When her ex-boyfriend unexpectedly showed up at the party, she put on a brave, neocortex smile but her eyes revealed her inner distress.
"Have a nice day" the receptionist said for the 30th time that day through her plastic, neocortex smile.