Chaos engineering
Chaos engineering deliberately injects failures into production (or production-like) systems to validate they recover gracefully. Pioneered by Netflix with Chaos Monkey in 2010, it catches reliability assumptions that only show up under failure, single points of failure, missing retries, runaway memory under partial outages, before real customers hit them.
Mature chaos programs run scheduled experiments (kill a node, throttle a service, drop a network segment) against staging or off-peak production. The discipline is keeping experiments scoped + reversible, chaos engineering is not 'break things for fun.' Each experiment has a hypothesis ('the system will reroute traffic within 30 seconds'), success criteria, and an abort path. Tools: Gremlin, AWS Fault Injection Service, LitmusChaos, Toxiproxy.