Around 2015, teams broke monoliths into event-driven systems. Publishers sent messages to queues. Subscribers processed them independently. One failing service wouldn't cascade through the system.
Then debugging broke. A single event triggers multiple subscribers, each spawning separate workflows. When something fails, you can't trace backwards. The linear request chain that made troubleshooting straightforward disappeared. One financial services platform saw throughput jump from 10,000 to 25,000 transactions per second. Accuracy dropped from 99.99% to 99.7%. Annual reconciliation cost: $2.9 million.
You built for resilience. You lost the thread of causation.