Tell me about a difficult technical problem that required sustained investigation before you found the root cause. Explain the symptoms, how you narrowed the search, the evidence that confirmed the cause, and the lasting fix. ### Constraints & Assumptions - Choose a problem that was genuinely ambiguous rather than a known issue solved by one search. - Distinguish hypotheses from facts observed during the investigation. - Explain your own work while crediting collaborators. ### Clarifying Questions to Ask - Could you reproduce the issue reliably? - What logs, traces, tests, or controlled experiments were available? - What customer or operational impact constrained the investigation? ### What a Strong Answer Covers - A disciplined timeline from symptom to competing hypotheses. - Instrumentation or experiments that eliminated possibilities instead of random changes. - A causal confirmation such as a minimal reproduction or controlled before-and-after test. - Safe mitigation while investigation continued and clear escalation when needed. - A durable fix plus monitoring or regression coverage for the failure mode. ### Follow-up Questions - Which hypothesis consumed the most time, and why was it reasonable? - How did you know the final change fixed the cause rather than masking the symptom? - What evidence would now detect a recurrence sooner?