My experience is that everyone thinks their defensive controls are air tight until inevitably they're going through a post-mortem on a failure where someone says, "Whelp...Murphy's Law..."
Dare I say that most software engineers literally plan to hit Murphy's Law?
If you build websites, and you never get hit by Murphy's Law, it could mean you are being too conservative.
If you build bridges, your job is to make sure you never get hit by Murphy's Law.
To your comment, it ultimately comes down to some tolerance and that's exactly what I struggled with.
Nobody cites Murphy's Law when you're in a third world country and the power goes out...for the 100th time in a day.
I can think of some systems that are really fault tolerant, but I can't find an example of some machine that's been flawless despite amazing engineering controls.