In two startling occurances Georges St. Pierre was TKO'd by Matt Serra, and Patrick Cote was knocked out 9 different times by Alessio Sakara. Both men have never been KO'd before or since, and you'd say that they have iron chins, but they have a weakness when it comes to the back a the head.
Any Belfort opponent, Joe Lauzon in the Florian fight and a million other MMA fighters have had the back a the head experience, and took the shot, but the Quebecois don't (for whatever reason) take those shots well and a KO is right around the corner.
Johnny Hendricks, don't need no advice but mine. Back a GSP's head is his weak spot, bro.