A terrible, horrible, no-good, very bad day at Slack
TL;DR First a performance bug was caught during rollout, and rolled back within a few minutes. However this triggered their auto-scaling of web apps to ramp up to more instances than a hard limit they had. This in turn triggered a bug in how they update the list of hosts in their load balancer, causing it to not get updated with new instances, and eventually go stale. After 8 hours the only real remaining instances in the list were the few oldest ones, so when they then scaled down the number of instances again, these old instances were the first to be shut down, causing the outage, because the instances that should have taken over were not listed in the stale load balancer host list.