React Guideline: Effects — Clean up effects. Description: Return cleanup function for subscriptions timers. Do: Return cleanup function in useEffect. Don't: No cleanup for subscriptions. Good Example: useEffect(() => { sub(); return unsub; }). Bad Example: useEffect(() => { subscribe(); }). Severity: High. Docs: https://react.dev/reference/react/useEffect#connecting-to-an-external-system.