I put two dollar coins into vending machine #1, because I wanted to get rid of them. Pushed a button, nothing happened. Pushed another button and got the wrong soda.
I decided to just pay again rather than make a big fuss, so I paid with a dollar bill. Then I realized I didn't want to get the wrong soda again, so I asked for my change back...and it returned my dollar coin.
So now I only had quarters and a dollar coin, and vending machine #2 doesn't accept dollar coins. So I try the card reader. I swipe, choose my soda, and press 'complete.' It doesn't give me anything and says it charged me $0.00. This happened the second time I tried it, too, before finally giving up.
Here's the kicker: That night, I found two $5 charges on my card from the vending machine!