vending machine in my junior high -
they had too many early-morning incidents of kids gathering outside and breaking into the machine, so they covered them with locked gates except for the buttons and retrieval hole.
well little did they know, a foot fit perfectly in the button-hole, and a swift kick to the mechanism at just the right moment (as the thing is turning), would cause the screw to continue turning for about 2 minutes,completely dispensing all of that particular product
we kept it on the down low for a while for fear that would fix it.. then too many people found out about it and they replaced the machine =/ lasted about about a month though the vending machine guy was probably like wtf
oh I also should point out a story about the bottled-water vending machine at my work.. I was buying my girlfriend a water, she happens to work at the same office. well i insert the three quarters, press the button.. and it dispensed the water, and then spit my money back out. I put it back in, and got another water... and my 75 cents back again.

satisfied with just that, I told her about it.. she proceeded to go out and retrieve like 10 free bottles of water from this thing. I was like WTF you are going to get in deep sh!t.. well fortunately nothing happened and she later left the vendor guy a note taped to the machine with some bills