Preface:
I searched for a few things, but couldn't really figure out if this has been brought up before.

Issue:  Mob's character model 'parts' (arms, legs, body, head) are not displaying properly at times, resulting in some very unique graphical oddities.
As you can see in the included screenshot a "Zombie Villager" was just two legs walking on top of one another.

Resolution:  Unsure, however, alt tabbing DID fix the issue, so it appears it may be some sort of shader cache issue.