vending machine logic

So I just bought a soda from a vending machine at work. I'm feeling appreciative that dollar bill readers have improved greatly since their first inception when the first dollar I was inserting definitely would not have been accepted... at least not without multiple attempts while standing on one foot singing the star spangled banner. The display reads $2.00 (because a 20 oz soda costs $1.50 these days!) and I press the button for Coke Zero (I normally would choose Diet Coke, but figured I'd try Zero) and after a few seconds of vending machine "thinking" noises, out comes... my second dollar bill. Really? I get annoyed when I press the money return button and it gives me coins instead of my bills back (except when I use that as a feature to get change for the parking meters). I ended up putting the same dollar back in and pressing the other button for Coke Zero and ended up with a 20 oz bottle of refreshment. Of course now I wonder what would have happened if I had pressed the same Zero button again...