Being the command guy I am, I was toying with custom heads. However, I couldn't help but notice a black box around the 'cooldown' box and the dual hand box. This only happens when I have a custom head in my inventory somewhere (EXCLUDING my head). Video is attached.

Thanks,
ThePTGuy

(This might be something to do with my OptiFine, if it is, sorry!)

UPDATE: This happens with the player skull in the creative menu, too.