If a tiny problem can cascade like that, it seems that there’s a systemic logistics issue going on here, the problem wasn’t the nail it was some high-level problem in the overall organization.

One nail is small and unimportant but the general problem of getting enough nails is a big important one.

And anyway, the messenger also could have been shot, the horse could also have tripped on a rock, the battle could have been lost even with the message getting through. If their plan hinges on everything going right, the kingdom has put themselves in a position where they don’t have any small problems, just big ones.

I think the argument the poster is making is as root cause analysis.

The root cause of the messenger failing was the missing nail. Sure it could have been many other things, but in this case it was the nail. And if it was a pitched battle that was narrowly lost by one message, sure, they could have won or lost because of a dozen other factors, but in this case it was the missing message. There are likely many other important things to worry about, but in the system as it is today, it failed for want of a nail.

Plenty of large engineering outages were because of single keystroke typos. Should these systems be less prone to human error? Of course. Are they? Some of them are, but right now some of them aren't.

The point being made is that small things can be important if other things go wrong. We should fix the other things, but often they are much harder to fix than the small thing. And really, we should care about both, since humans are capable of that.