Whenever I teach someone to shoot I stress the importance of keeping your right elbow up in the air so that your upper arm is parallel to the ground and you form a pocket for the butt of the gun. If you let the butt slide out onto your shoulder, or worse, your bicep, bad things will happen. This hideous photo shows just what those bad things look like.
This nauseating injury occurred halfway through a sporting clays match. The owner of the hematoma was shooting an inexpensive 12-gauge over/under, and about halfway through the match felt something very bad happen in his shoulder and upper arm. What he felt was a) a torn upper pectoral, b) torn upper bicep, and c) mangled rotator cuff. Not to mention all the nifty purple blood underneath the skin. His right arm is largely useless and will be for a while. And as I can tell you from personal experience, there are few things that hurt more than a screwed-up rotator cuff.
The lessons to draw from this are:
Always mount your gun correctly.
Never shoot sporting clays.
Never shoot a cheap shotgun.