The incident with no onset spike and no pager: precision bled out over weeks while every dashboard stayed green. How to run a blameless postmortem when the incident has no clean start time, the root-cause classes that hide this long, and the action items that actually prevent the next one.
Updated Sep 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
The incident with no onset spike and no pager: precision bled out over weeks while every dashboard stayed green. How to run a blameless postmortem when the incident has no clean start time, the root-cause classes that hide this long, and the action items that actually prevent the next one.
The structural insight that separates seniors is that traditional postmortems assume a discrete onset, and silent degradation has none, so 'when did it start' is itself a finding about your detection lag, not a timestamp you can look up. Interviewers listen for whether you distinguish detection failure from the underlying defect, because the most important action items usually fix detection, not the model. A weak answer reaches straight for 'retrain'; the strong one first asks whether the degradation was drift, an upstream feature change, a skew bug, or a label-pipeline problem, because retraining only fixes one of those.
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