Everyone has seen that airplane bathrooms still have ashtrays, despite smoking being banned on flights for almost a generation now. Engineering Infrastructures For Humans has a great blog post explaining why (spoiler: because some people will still smoke). Even greater is the takeaway for anyone who builds anything:
You don’t engineer your systems with the belief that none of your computers will ever break. That’s insane; you KNOW they’re going to break. So don’t assume that your users will never break the rules. Build in graceful failure as often as possible, whether you’re designing a user interface or a security policy.
Airplanes are actually quite redundant in almost every respect from human error to mechanical failure. They are, generally speaking the gold standard for engineering.