A wizard’s staff poked out. The chieftain saw the knob on the end. “Now, then,” he said, pleasantly. “I know the rules. Wizards aren’t allowed to use magic against civilians except in genuine life-threatening situa-” There was a burst of octarine light. “Actually, it’s not a rule,” said Ridcully. “It’s more a guideline.”
— Lords and Ladies, Novel by Terry Pratchett
There are a lot of programming principles such as DRY, SOLID, KISS etc. Just keep in mind they are not rules, they are more guidelines.