Interesting. I also work for a cloud provider. My team work on both internal infrastructure as well as product features. We take testing coverage very seriously and tie the metrics to the team's perf. Any product feature must have unit tests, integration tests at each layer of the stack, staging test, production test and continuous probers in production. But our reliability is still far from satisfactory. Now with your observation at AWS, I start wondering whether the coverage effort and different types of tests really help or not...