I've witnessed someone hit by a ricochet .50 BMG round before, and in that instance the target was an aluminum engine block. The return trajectory was approximately 45 degrees, and about 60-75 yards back. Impact was on the leg, with denim jeans worn. The slug did not penetrate the material, and I don't believe there was any injury other than bruising. The recovered projectile (fell to the ground) was quite hot to the touch, and looked to have held together rather well (maybe 500-600 grains), despite the large flattening and bending of the tip.
All I can say about ballistics, is that absolutely anything concievable will sooner or later occur, if you fire enough rounds downrange.