DNS TTL cache split
Twenty minutes after a planned DNS change repointing api.example.com to a new ingress IP, you get partial outage reports: roughly 40% of users can't reach the API while 60% are fine, and the split doesn't correlate with region. Your new ingress is healthy and serving the 60%. Some clients resolve the old IP (now decommissioned, connections refused) and some the new IP. The old record had a TTL of 3600s. A handful of large enterprise customers behind corporate resolvers are heavily represented in the failures. Triage and stabilize.
What a strong answer looks like
Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.
0:00 of about 35 min
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.