Why is "For Want of A Nail" considered impossible in real life?

Seems like something that would've happened in many points in history, but real life examples are banned. I'm not asking for them to be unbanned, but may someone please educate me as to why it's considered impossible?

Probably because things in real life only happen once with no possibility of a do-over, which makes it impossible to tell exactly how something would have happened otherwise.

Unless someone gets Time Travel to work in RL, or proves the multiverse hypothesis, it's impossible for any examples of that trope to occur.

edited 18th Jan '18 1:43:51 PM by Fighteer

So am I mistaken? I wasn't under the impression that For Want Of A Nail required at least two instances of the same scenario with a slight variation to play out.

It requires that we know how the scenario would have played out if it weren't for that small detail, whether by having the event actually happen twice (thanks to time travel or a "what-if?" spinoff), or having the narrator explicitly point out the consequences of the missing nail.

For example, in the poem from which the trope gets its name, we are told that the fall of a kingdom can be traced to a missing horseshoe nail. But if a similar scenario happened in real life, we would have no way of knowing that the nail was the catalyst of the kingdom's defeat: what if, even with the successful delivery of the message, the battle was still lost? What if they did win the battle, but ended up losing another one in the future? Without a narrator telling us how it could have gone, it's impossible to know what might have happened.

Oh, okay, that makes a lot of sense when you explain it that way. Thank you!

Then I suspect that I might've seen this trope being misused in some pages...