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Behavioral · Core Behavioral

Mistake in a pipeline (STAR)

Mediumbehavior-02
STARmistakeidempotencyownership

Question

Tell me about a time you made a mistake in a data pipeline. What happened and what did you learn?

Solution

Use STAR. Pick a real mistake (internship, project, or personal pipeline). Own it without drama.

Framework

  • S: What broke (wrong join, duplicate load, bad partition, missed null check)
  • T: Your ownership of the job / table / dashboard
  • A: How you detected it, fixed data, and prevented recurrence
  • R: Impact contained + process improvement

Example outline

Situation: Daily orders job loaded the same partition twice after a retry, doubling revenue.

Task: I owned the load step and the downstream dashboard refresh.

Action:

1. Confirmed duplication with COUNT(*) vs source and row hashes 2. Paused the dashboard refresh and notified the analyst 3. Rebuilt the partition with delete-then-insert (idempotent load) 4. Added a uniqueness test and made the load overwrite the date partition 5. Documented retry behavior in the runbook

Result: Metrics corrected same day; next retry did not double-count. I learned retries without idempotency are dangerous.

Fresher tip

If you never broke prod, use a project: "In my course project my join exploded row counts; I fixed cardinality and added a row-count check." Honesty + learning beats fake hero stories.

Interview tip: End with the prevention (tests, idempotency, alerts), not just the apology.

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