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A service you deployed at a customer corrupts a record intermittently under concurrency. How do you find it and fix it?

It reproduces once in a thousand runs, never on your laptop, and the customer's data is wrong. This question separates engineers who name the race and reach for the right lock from those who sprinkle mutexes and pray.

Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.

It reproduces once in a thousand runs, never on your laptop, and the customer's data is wrong. This question separates engineers who name the race and reach for the right lock from those who sprinkle mutexes and pray.

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Next in this trackDesign the integration API a customer's engineers can adopt without it breaking on them: REST vs GraphQL, versioning, pagination.PremiumNext in this trackHow do you design a customer deployment so their team can run it without you, and you make yourself obsolete?PremiumNext in this trackDesign a real-time collaborative coding assistant: stream completions under 200ms for 10k concurrent usersPremium
FEDITOR'S NOTE

Look for the read-modify-write race being named precisely and the optimistic-vs-pessimistic choice made on contention level, not vibes. Strong candidates know optimistic (version column + compare-and-set, retry on conflict) beats pessimistic locking under low contention and avoids the deadlock surface entirely. Probe with 'now two transactions deadlock' to test lock-ordering knowledge, and 'it only happens at the customer, not in your tests' to test how they reproduce a concurrency bug they can't easily attach to.

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