A divinatory card system is created that is unreasonably effective. However, it turns out that this is because the cards cause the foretold future to occur (rewriting history if necessary), not because the cards predict what the future would have been -- and this distinction occasionally becomes more than merely abstract, as a (randomly chosen) divination of the near future sometimes requires erasing the act of divination itself. More mundanely, events surrounding people using these decks begin to occur at a rate more closely approximating their probability of being predicted by the deck itself, rather than their natural probability (making fortune tellers magnets for very strange events). Eventually, an ancient race of aliens come to earth in order to investigate the sudden increase in entropy being produced. Unfortunately for them, people have begun using these cards to play a poker-like game -- never realizing that every fair shuffle of the deck is scrambling important parts of the fate of the universe itself.
