Someone bought a gift card, then this person made stickers of the bar code from his card. Then the person used this stickers to place over the bar codes of the cards in the store. Someone buys someone a gift card in the store and what they have funded is the card from the person who placed the sticker.
Whoever came up with this is a genius, morally it is very wrong of course. I would be pissed if I gifted someone a card and it turned out to be empty