A successful job doesn't prove its result is complete. Compare source and destination at a stable boundary using counts, distinct identities, and control totals that match the business grain. A sum can reconcile while records are missing if one duplicated record has the same value as a missing one. Independent checks make that failure visible.
A backfill is another writer. If it shares a destination with live processing, define ownership, version rules, and the point when readers switch. Replaying input with append-only writes can double a metric even if each job succeeds. Prefer idempotent merges or replacement of a precisely bounded result partition. Validate a shadow result before publication and retain a rollback path. Late corrections also change denominators, so check a rate's numerator and eligible population together instead of trusting an unchanged percentage.