https://www.pagerduty.com/blog/the-discovery-of-apache-zooke... - an old postmortem that I still think was a masterclass in debugging, uncovering two bugs in ZooKeeper and two in the Linux kernel.

https://about.gitlab.com/blog/2017/02/10/postmortem-of-datab... - thousands of GitLab projects permanently lost. A stark reminder of the importance of database backup testing procedures, because even multiple layers of backups can fail in a black swan event.