This (see ivan37's writeup about cheating dollar bill validators) is part of the reason why the United States vending industry lobbied so hard to get the dollar coin, both in the 1970s and more recently. (That, and the fact that dollar bill validators are much more expensive than the mostly-mechanical coin mechanisms, and much more trouble to maintain... Tell me, how many times have you seen a machine whose dollar bill validator was broken? Okay, now how often have you seen a machine that woiuldn't take coins, or even a specific denomination of coins?)

There are some dollar bill validators that are designed to prevent this -- I've heard of ones that have a blade inside that is designed to cut off any trailing bit from an accepted bill -- but of course they're more expensive, and probably only used on machines that take in more money.

My father told me a story of a soda machine that was ripping itself off -- coins were falling through to the change/coin return slot, but still registering as having been inserted. (I wonder if somebody used knarph's salt-water treatment on it.) Anyway, after inserting and receiving back the full price of a drink, he found that everything in the machine was sold out (I wonder why...), but he pressed the coin return and got repaid an additional amount equal to what he put in. I'm sure that if he was willing to stand there messing with it for so long, he could have gotten all the money out of the machine.

Later, when I was visiting him, the same machine had broken in the same way again. He pointed out to me two giggly girls spending an unusually long time in front of the machine, and then one of them went in the store and came out with an empty bag, which the other one filled with all the free sodas they'd been getting.

When they left, we went over and got ourselves free cokes. Just one each -- we're both too honest to do any of the stuff described in this node, but not so honest we'd pass up a free drink when we know the machine is giving them out.