Firstly your shoulder would follow the rotation for as long allowed. Then something would be dislocating, but whether it would be the head of radius or your shoulder I cannot tell. From there you get a whole lot of soft tissue injuries.

If you'd somehow continue to pronate (rotate your hand "inwards") and fixated your upper arm, you'd probably get a fracture of the proximal ulna or distal humerus, as they are rotationally fixed to eachother.

Note, I have not tested this. This is only my intuition, as someone who knows a bit of anatomy and medicine.