A few years ago, I shipped what looked like a textbook, harmless API update. The endpoint returned valid JSON. Every single local test passed with flying colors. We hit deploy, dusted off our hands, and went to grab coffee. Ten minutes later, Slack exploded. The frontend team was reporting errors across the entire app. The culprit? A seemingly minor refactor: I had changed a property named name to fullName, and dropped another field entirely because "surely nothing is using this anymore...