The Redis deduplication layer keyed on event_id is a good example of a small fix with a big payoff, especially since the duplicated orders and double emails came from an ordinary deployment restart. Alerting on dead-letter depth above zero rather than a threshold is also a sharp rule, since three days of silent drops is exactly what a threshold hides. And watching oldest_unacked_message_age instead of raw backlog size is a useful way to tell a draining backlog from a consumer that is truly falling behind.