"A story is not a machine that does what you tell it. A story is a beast with a life of its own. You can create it, shape it, but as the story grows, it starts wanting things of its own. Change one thing, and you set off a chain reaction of events that spreads through the whole thing. The characters have to be true to themselves. The events need to follow a logic that fits the story. A single flaw and the magic is gone. The story dies. - Alan Wake" - Sam Lake

Established rules and constraints must be consistent throughout the story, otherwise nerds on the internet with nothing better to do will call you a hack. See: the new Star Wars. Force healing, my ass.