A cache key ate 99.9% of my records and the pipeline looked green

The worst pipeline bug I've shipped didn't throw. No stack trace, no failed job, no alert. Every run went green, finished inside its window, and wrote roughly one record out of every thousand it was handed. The job's own metrics said it was healthy, because the job's own metrics counted runs, not rows. It took a business user asking why a campaign list looked short to surface it.