Well depending on the brand of soda machine there are a few hacks that aren't malicious. For ones that I've seen, I start counting from the top. Press buttons 4,2,3,1 and it will tell you inventory inside the machine. There are other button combos like this. Still I believe it takes a key to open these models. I was just more interested in the fun things you could get the machine to tell you.
Ah ive done the facebook thing tooo, Ive tried to do the paypal thing from unregs (that ive checked latter and have found to have paypals, but it always wants secert answers to questions when you try to request lost password).
I used to get folks Yahoo messenger IDs to, and mess with those.
and for hacking a soda machine, depending on the type of dollar bill dispenser, pre 80s you cut a triangle into the dollar bill about inch into it.
after 80s, you tape about 5 feet of tape to your dollar bill, allow it to eat it, then yank that mofo back out.