January 2021, Fastly CDN major outage. Root cause: configuration change during software deployment interacted unexpectedly, causing CDN to stop responding entirely. Thousands of sites—GitHub, Twitch, Amazon, The Guardian, Financial Times—went down for ~49 minutes. Fastly's CEO stated in postmortem: "didn't sufficiently validate changes before deployment."
Three CloudFormation tools prevent such incidents. Change Set dry-runs changes before deployment, Drift Detection catches reality-code gaps from manual changes, and Rollback Trigger auto-reverts to previous state if service anomalies appear post-deployment