"Failures are rarely caused by isolated errors but by the unexpected interaction of multiple factors.
Reason is pointing at the failure mode behind most confident postmortems: the urge to locate one cause and declare the problem solved. Real failures are chains, not single points. Single-cause stories are comforting because they make the fix feel small.
In software, the chain can be ordinary: a config change, a latent assumption, a retry loop, and an alert that arrives late. Each link is defensible. The interaction is what breaks you. If you only fix one link, the chain rebuilds itself somewhere else.
Write the interaction story before it happens. Describe how two or three reasonable choices combine into a bad day, then design one guardrail that breaks the chain. You are not predicting; you are refusing to let the first weak link stay invisible.