If you build software long enough, you eventually ship a bug that isn't a typo or a logic error — it's a default . Specifically, it's the answer to a question nobody asked out loud: "What happens when this check fails?" That question has a name in security engineering: fail-open vs fail-closed . The door analogy Picture a badge-controlled office door. Normally you need a card to get in. Then the power dies and the lock controller crashes. In that instant the door has to "dec...