In a nutshell: the world of a fictional universe isn't fixed beyond what the author has revealed to the reader. So, when faced with an impending Off the Rails event, the author can subtly tweak the facts the reader hasn't discovered yet to suit the changed plot. Improvisation masquerading as planning, if you will. For obvious reasons, this kind of plot event is pretty much impossible unless the work in question is either interactive or is written at such a scale that the author can change plot events based on what (s)he hears in the fandom. See also the Useful Notes regarding Schrodingers Cat.