EJECT CASH button for Vending Machines.

[Frank_Stein]

I like letting money get returned. But make it have a high chance to mess up and/or a give a reduced refund. Hitting refund for 5 might make a bunch of coins and broken wires spew out and 3 creds.
Once Discount Dan has your money, he's not too intent on giving it back.

If we wanted to add an extra layer of frustration to using vending machines, I'd rather it not make you lose a ton of money.

But having the machine sometimes reject cash for being too wrinkled, or the items doing that thing where they get stuck and the next purchase drops two would be fun.