Head Glitching in Call of Duty: Black Ops 2

Head glitching is possible in Black Ops 2 and every Call of Duty game because the first person animation and the third person animation do not match up.  This video shows how this works:

To see the details you should probably watch in 720p.

Essentially the third person's animation is hunched down compared to the first person's.  This makes the head and gun lower in elevation so that when directly behind an object the gun and head are not looking out over top of the object, but rather are below the surface of the object.