MUST USE when fixing a bug, diagnosing an error, stack trace, exception, crash, failing or flaky test, or any 'why is X not working / returning the wrong value' investigation in any language. Enforces root-cause-before-patching: reproduce, trace the failure backward to its origin, test one hypothesis, then fix once — instead of guarding the symptom. Also covers the 3-attempts-then-question-the-design rule.
