A power failure in one Google Cloud zone brought down British banks
Google cloud outage takes down banks
A power event in Google Cloud's europe-west2-b zone knocked multiple services offline, and UK banks including HSBC, Revolut, Lloyds, and Bank of Scotland went down with them. Hacker News commenters are stunned that major institutions weren't spread across multiple zones, and note that data-sovereignty rules may have pushed British firms into a single GCP region. Others point out that multi-zone failover is rarely tested, so it barely exists.
Much as with backups - if you don't test them, they don't exist. Everywhere I've worked has had multi-AZ infrastructure. I have never worked anywhere where the failure of an AZ has been simulated regularly, with second-order effects mapped out.