A non-principle is a rule of thumb/law/heuristic/etc. of software development which is similar to a principle but actually is not one because it does not help distighish good solutions from bad solutions.
Non-principles may carry valuable knowledge althougth they cannot be applied like principles. Non-principles often describe phenomena or effects and give them a memorable name. But they do that without any justification.
See the list of non-principles in this wiki.