Two presenters at Black Hat 2009 just demonstrated their ability to hack into parking meters in San Francisco (and theoretically anywhere with this kind of system) to give unlimited money on their parking payment cards.
The two figured out how to do this by intercepting the types of signals that get transmitted on a working card, then programming a fake card to reproduce those same signals.
They found that the card has a stored maximum value and only writes how many times the value has been decremented.
In essence, they found a way to give themselves free parking for as long as this system is in place. They declined to detail how exactly you can reproduce this, because they don't want people ripping off the city of SF, so their sole purpose is to get SF to fix their meters to that this can't happen. [Hackaday]