I withdraw an enormous package that I can barely tug out of the slot. It's all wrapped up in thick brown paper, and takes me like 5 minutes to open. Inside, there's nothing. I bang the vending machine in frustration - surely this is a mistake? But the machine refuses to divulge anything else - so this is what I was meant to get.
I scribbe a little note on a piece of scrap paper that says "This machine is dodgy, you'll never guess what comes next", wrap the note around a coin and shove it down the coin slot.