I think this is the biggest problem with Black Ops 3 so far and Treyarch really need to look at fixing it in its current state. As this is a combination of netcode and map aesthetics; I'm not sure if they will be able to fix it to any standard without having to practically overhaul map design or rewrite the entire character code but it's bloody annoying playing against it particularly on maps like Infection, Metro and any others with decent cover elements. It's been an minor gripe in other Call of Duty titles previous but it just feels so more prominent in BO3.
If anyone is unsure of what headglitching is; basically if you're looking at someone behind cover and all you can see is their head but from the opponent POV; they have their gun and torso seemingly exposed. Bullets don't fire from waist or chest level either - it's actually head level which is why you can get shot by someone in cover without seeing them from the head down meaning you have a small target to hit and they can kill you full view.