It seems to me that someone forgot to paint the 
black shadows under his chin, so they painted a 
seperate piece of acetate (plastic) black and 
taped it to the back real quick so that its 
correct on film.. I'm guessing they didnt notice 
this til right before they shot it, otherwise 
they would have probably just painted the actual 
cel, but they didnt (probably because they didnt 
want the wait for it to dry)....not positive 
though.. just a theory ^_^