The behind the neck pulldown is probably the most common high-risk exercise I see at the gym. This exercise puts stress on your neck, back and shoulder. It is obvious in the picture above that he is leaning forward and hanging his head, which causes a great deal of stress on the vertebrae in the neck. In addition, this position causes a great deal of external rotation in the shoulders, which overstretches the connective tissue of the shoulder. Eventually, especially with heavy weights, this will lead to a rotator cuff injury.
This is an easy fix because pulldowns in front of the neck are just as effective. If you are looking to add variety or difficulty try this exercise sitting on a stability ball or kneeling on a BOSU trainer.