The Upstream Schema ChangePipeline · easy · ~10 minThe CRM team renamed user_id to customer_uid in their export and added two columns — announced only in their own team channel. Downstream, your ingestion didn't fail: the landing model does SELECT *, so the new schema flowed straight through. The join in dim_customer then matched nothing — user_id no longer exists downstream — and BI silently showed 100% 'unknown customer' revenue attribution for two days.No task failed. Fix the contract, not just the column.Evidence

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Why did nothing fail while everything broke?

The layered defense for next time?

'Data contracts' can sound bureaucratic. Make the pragmatic case using this incident, interview-style.