In my other life (the one that's currently eating everything) I test software. The programmers who curse me daily often tell me they're done with whatever they're working for, and, in theory, this is true.
The problem with the strange and mystical land of Theory, is that it's not this world. In Theory, everything works to spec. Plans can be followed, and software is delivered for testing without weird glitches and weirder bugs (I should probably mention that I'm well known at work for finding the weird bugs that do catastrophic things to the system, but need the stars to be aligned and the moon in the correct phase and... well, you get the idea).
As writers, we run into problems with the land of Theory from two directions. Your nicely planned, outlined story changes its mind and twists, or worse, you write stories where everything works the way Character X planned it.
Readers of course immediately know that the latter situation can't ever happen, because every time something like that happens to them, they're looking around suspiciously, searching for the huge disaster that they just know is waiting for them. Because plans never go according to plan. It's a Rule. One of those ineffable ones, the ones writers always seem to end up effing because we just can't help ourselves.
As for stories twisting... Let's put it this way. The last time I tried to write romance, I got the piece with the working title of Evile Elves. Because they're not just evil, they're Eeeevile. The less said about my attempts at fanfic the better. I can't even manage standard epic fantasy. It's just as well I enjoy writing the stuff because editors take one look and start backing away while being very, very nice to the crazy lady.
The land of Theory. Wonderful place, that. Everything does what it should. What writing theories have you run into that don't place nice with reality?