Provider rate limit on fan-out
Your notification service sends SMS via a third-party provider (Twilio-style). After a product launch, users report missing texts. Your logs show a wall of HTTP 429 `Too Many Requests` from the provider with a `Retry-After` header, and your provider dashboard shows you've hit your account's per-second message quota. Your service fans out one API call per recipient with no client-side limiting, and you just retry immediately on 429. The launch announcement targeted 500k users in a 10-minute window. How do you triage and get messages flowing without burning more quota?
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.