This experience needs JavaScript. In short: one Thursday on a feed-ranking team. Pair every goal metric with a guardrail; read a clear negative experiment result as a result; roll a suspect model back before you diagnose it; ask whether a new feature is computed the same way in training and in serving; put a cheap first pass in front of the heavy model and keep latency headroom for the evening peak; and turn the failure you caught by feel into an alarm.