The holes that kill you are the ones you never tested

Every time something goes down, the first instinct in the room is to add a layer. Another replica. A second region. One more health check in front of the thing that broke. It feels like progress, and the Swiss cheese model hands that instinct a lovely picture to point at. You know the diagram even if you've never heard the name. Stacked slices of cheese, each slice a layer of defence, every slice riddled with holes. An accident only makes it through when the holes in all the slices happen to line up. James Reason built it to explain how hospitals and aircraft fail, and it's still the best tool we have for explaining why a system with five safety nets can still face-plant. The picture has a side effect though. It teaches you to count slices. And counting slices is mostly the wrong job. Take redundancy, since that's where slice-counting does the most damage. The model says two of everything beats one. Two servers, two regions. That holds right up until both copies share a single reason…