At my highschool cafeteria we'd use these RFID charge cards which you could load cash into. Now, I didn't have the means to access an RFID reader to mess around

At my highschool cafeteria we'd use these RFID charge cards which you could load cash into. Now, I didn't have the means to access an RFID reader to mess around with it, but turns out that there were other problems. Being the absent minded person that I am, I'd often lose these cards, pay some extra amount to get a new card, and then find the old cards in my pocket, in a bag etc. The old cards wouldn't work in the main cafeteria but they'd transfer the cash amount you had into the new card (minus the card fee) for you. However, they would work in the coffee vending machines. It turns out they actually stored the cash value in the cards! So, I ended up loading a large sum of money in a card, "losing" it, and me and my friends had free coffee and hot chocolate for the better part of the most academically challenging two years of my life.