Butterfly effect.

As Jon himself was perhaps the best proof, a single wish can change things completely out of proportion with the benign intentions that spawned it. A wish is a change imposed upon reality, and all of its ramifications are not always immediately apparent. When Jon uttered his careless wish, the stone, in an instant that lasted less under a microsecond, calculated. It looked at all the "interesting" situations it could perceive for its user, and at the energy expenditure it would have to output to achieve them, looking for an optimal "cost/effect" ratio from among trillions and trillions of possibilities. Until finally, it settled on something small, something simple. Someone, somewhere, was about to make a wish of their own. That wish, if it were to become true, would eventually lead to an interesting outcome for Jon. The stone was about to grant that wish.