Postmortems

What does a postmortem need to produce, and what is "the root cause"?

Whether the candidate can hold contributing factors rather than a single cause, and whether the outputs actually change the system.

The situation behind the question

An outage lasted 90 minutes. The report says: "Root cause: an engineer deployed a change with an untested migration. Action item: be more careful with migrations." The migration in question had been reviewed by two people and the pipeline offered no way to run it separately from the deploy.