The problem: a small change in the checker’s validation rules. An innocuous refactor renamed a field, tightened a regex, or reinterpreted a truthy value. The checker began to treat certain valid requests as invalid. Worse, instead of returning clean, debuggable errors, it normalized rejected payloads in a way that silently dropped critical fields. Some consumers received success responses with degraded behavior; others saw weird partial processing; downstream systems received corrupted events. The result: cascading failures, lost messages, and a production incident that looked like a distributed puzzle.