They were not around for very long but there was a soda machine that had a conveyor belt that drinks would drop down onto and then be pushed out of a flap door into the bucket. The machine could detect when a drink was actually vended but didn't seem to be able to detect when someone was holding the door shut. You'd pay for a drink and hold the door shut while the machine tried to push your drink out the door. Since the door wasn't opening, the machine would think that no drink ever dropped down onto the conveyor belt so it would mark that specific drink number as out of stock and return your money. Except your drink selection was still sitting on the belt. You'd pick a new drink and pay for it but this time not holding the door shut, two drinks would come out for the price of one. You could do this quite a few times over to either get a couple free drinks or just jam the machine. Out of all the vending machine "hacks" I learned about in middle school, this was the only thing that ever worked and only for a short time before they got quickly replaced.
The new machines use either a basket to hold the drink instead of a conveyor belt or some sort of shield to prevent you from messing with the flap door until it knows a drink has been vended.