When Jon had to go to school in the morning, he knew he couldn't take the heavy stone with him. So he made his wish: "I wish that nobody other than me could come into my room today."
This would have worked fine if the strain of the heavy stone on the floorboards had made them finally break, and the stone had collapsed a part of the floor, fallen through, and landed in the living room below.
Needless to say, it was rather easy to notice, and just ripe for people to make accidental wishes on....
