buy one vend one free
When a person goes to a vending machine intending to buy an item and that item is barely hanging on to the barricade that prevents it from falling. The person then buys the item and two items fall from the same row.
Dude, I went to go buy some sour patch kids and the idiot before me only had a dollar and his sour patch kids got stuck. I got to buy one vend one free.