Hacking a Bicycle Rental System
CallABike offers bicycles to rent in several German cities. You register with the company, find a bike parked somewhere, and phone the company for an unlock key. You enter the key, use the bike, then park it wherever you want and lock it. The bike displays a code, and you phone the company once again, telling them this code. Thereafter, the bike is available for the next person to use it. You get charged for the time between unlock and lock.
Now read this site, from a group of hackers who claim to have changed the code in 10% of all the bikes in Berlin, which they now can use for free.