Tell me about a time you dove deep to find a root cause

Describe a hard problem you had to investigate in depth, such as an intermittent failure where the obvious metrics looked healthy. Walk through the symptom, the hypotheses you formed, the evidence you gathered, how you knew you were looking at the right things, how long it took, and how you proved the fix addressed the real cause.

Expect several follow-ups on the technical details. A strong answer shows your own reasoning step by step and what changed afterwards so it would not happen again.