What’s scary to me is how very few people know the correct way to lift their arm up overhead.   

The humerus tends to rest in an internally rotated position. If lifted above the shoulder like that the head of the humerus will bump the acromion process eventually causing impingement.  Years of this repetitive motion (reaching for stuff, handling baggage in overhead bin, freestyle swimming, stretching with bad form, poorly aligned down dog, lifting weights with bad form) leads to injury and ultimately degeneration of the “rotator cuff”.