Principle: If a game requires a strategy guide or walkthrough to complete, it's broken. If the game may be completed without one but nevertheless provokes a not insignificant number of players to consult one, it's just as broken as before.
Rationale: Completing a game should require no external clues. A player's failure to figure out a particular puzzle without outside help is actually the designer's failure to provide sufficient clues within the game. The game world is missing essential information; the game is broken.
Exceptions: If a game mechanic explicitly involves the application of external information, this principle does not apply.