perhaps one would say such a thing as a means of reinforcing encouragement. Now I would never say something of that caliber without meaning it but I guess all people are cut differently.
If you know they're messing you got two options in my opinion:
Option A)Just read it as a meaningless thing and not worry
Option B)Ask the person if they are being truthful if they say no or laugh(probably nervous) turn around and then haul back and punch them in the nose.
Don't aim for the mouth as you don't want to get any scars on your knuckles.