What is the worst thing that can happen in a vending machine?

Many of us have been there, you put your money into the vending machine, press the button and the item that you have chosen gets stuck inside the machine. Normally a small knock on the side might work, but in this situation the thirsty consumer decided that surely another purchase would free the can of drink. Well, surely it has to be third time lucky then! What are the chances of this happening and more to the point, what would you de next?