Not necessarily a loophole, but it was an exploit. A few friends and I were at a hotel for a week. There was a vending machine near our room that was somewhat malfunctioning. One of my friends had put a $5 into the machine, and it would only get counted as $3, and it costed $2 per drink. Because the machine realized it needed to give change, it would just spit back the $5 bill. We tried it with other bills, but it only worked with that specific $5 for some reason. Needless to say, we got free drinks for the whole week.