Pipeline · medium · ~12 min
Mobile events arrive up to 6 hours late (offline devices, retry queues). The daily aggregation runs at 00:30 for the just-closed day — so it always misses the stragglers. Analysts noticed 'yesterday' is reliably ~3-4% low at 9am, then mysteriously correct a day later, because someone bolted on a nightly re-run of D-1. Now there are TWO numbers for every day depending on when you look, and nobody trusts the dashboard.
Design the late-data policy properly.