I'm familiar with a story of an ecommerce site that offered a discount on digitally delivered gift cards to encourage holiday gifting. But they didn't think to prevent people from buying more gift cards with those gift cards.
Nearly 20 years ago there was a large supermarket in the UK that had two 'points' offers on that intersected on bananas. Cue an enterprising few to buy all the bananas (with banana points), then quickly use all their points on other items.
My school ended up with a heap of free bananas from a parent :D
