Postmortem: How a Bun 1.2 Runtime Bug and Node.js 22 Interop Caused Our API to Return Incorrect JSON for 30 Minutes On October 17, 2024, our team experienced a 30-minute incident where our production Payment API returned malformed JSON responses to downstream clients, causing widespread payment processing failures for our users. This postmortem details the timeline, root cause, resolution, and preventative measures taken after the incident. Incident Timeline (All Times UTC) 14:45 : Deployment of Payment API v2.1.0 to production, upgrading the runtime from Node.js 22.1.0 to Bun 1.2.0 to reduce cold start latency. 14:47 : First customer reports of payment failures, citing "JSON parse error" in client logs. 14:50 : On-call engineer confirms API responses are returning invalid JSON: ~12% of responses have missing required fields, 8% have extra unexpected properties. 14:55 : Incident declared SEV-1, rollback to previous Node.js 22 based deployment initiated.…