Dual-write flag causes read-write skew
A flag 'write_to_new_store' has been ramping for the document service: it dual-writes edits to both the old store and a new store. At 5% for a week, fine. At 16:00 it's bumped to 50%. Within the hour, users report documents 'losing recent edits' on reload. The new store's write success is 100% and so is the old store's. But the flag is evaluated per-request, and a given document's writes can land on either store depending on which request evaluated the flag true. Reads always come from the old store. Triage, explain the corruption, then mitigate.
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 45 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.