One thing that I have always thought about when seeing a severed body part on a horror movie is arm pits. What EXACTLY is an arm pit? Its not so much a body part but more of a location. Would it be the arm cut off up high and then cut down through the shoulder and out underneath the "pit"?  Then its not so much a pit but just a chunk of skin with hair. 
For example, if you seen one fly across the room on a horror movie could you ID it? Certainly if a finger, head, or foot flew across the room in front of you it would be easy to decide what you just saw. You would say "hey, look a finger!" If am arm pit flew by you at mach speed all you could say was WTH was THAT? A bird? With hair?   
