I recently posted what I thought was a humble and polite comment that I saw the "chopped-off skull" error in someone's work http://www.deviantart.com/deviation/125939556/ , thinking that if someone told me that, I'd be glad for the help:
"I'm far from the best person to offer critique, so I hope you don't mind me saying this..
Your drawing has the "chopped-off skull" error, which makes the otherwise great drawing of his features look weird.
This picture on someone's art blog might help you see it: http://honoriartist.livejournal.com/397703.html
And here's some text about it: http://www.karenika.com/book/brain.html
Hope you don't mind me