Cause and Effect
Principle: Avoid responding to the player's actions with behavior that is contrary to reasonable player expectations, as it breaks the implied "contract" between the player and the game designer. Players cannot form a useful mental model of the game world when things happen unexpectedly or without an apparent cause. Players should be able to understand the laws at work in your game's environment, and to make appropriate deductions based on their understanding of those laws. If unexpected or incongruent behavior is desired, players should ultimately be able, by virtue of the game's design, to reconcile such behavior with their operative mental model of the game world -- which by its nature is allowed to change as the game progresses.