Canary session token skew
v140 of the cart service auto-promotes from canary to 100% at 12:00 after a clean 30-minute canary. By 12:25 support reports a steady trickle of users whose carts 'reset to empty' after adding items. Overall add-to-cart success is 99.95%; only ~0.3% of sessions are affected. The canary used per-request random routing (a request could hit canary or baseline independently). v140 changed the cart session-token format. Triage and explain why the canary passed, 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 40 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.