Because what I've learned lately is this:
Mis-writing a chapter is like tearing a hole in your plot balloon.
You can move forward at first, even explosively so, but then you notice with some alarm that the rubber ceiling above you begins to sag. Yet you continue on until you realize, with something close to outright panic, that the balloon's fabric has begun to pool in great reams around your feet and press down against your head, because the air's seeped out, see?
It's gone, man, and you can't even see your way out. You have to pull at the rubber and rifle through its folds until you find the hole. Then you have to climb through it and torch the whole mess like you're the evil clown.
Still with me? We're talking about mis-writing one chapter, one that - like Will Elliott's psychotic clowns twisting J.J.'s arms - can force your story into doing horrible things and create one big MESS of a plot.
Unless you realize your plot misstep and correct it.
If you don't, if you ignore the plot signs and press ahead anyway, you could stumble into unholy places - the dark, airless vaults of bad story. The ones that contain a whole CREW of evilly sadistic mega-clowns who (instead of devouring your protag's goofy sidekick and providing some page-turning character development as planned) turn on the big top of your plot itself, taking a chain-saw to the tent posts, dismantling your story's trajectory as surely as the Langoliers Pac-Manned their way through yesterday's reality.*
But with all that big top talk, maybe incorrectly writing a chapter is more like a circus. A Pilo Family Circus.
Okay, sure. Elliott's clowns have NOTHING on Pennywise. But go on, release their dreadful calliope carnage into your MS and see what happens. Dare you.
All of this to say, I just realized where my plot mis-step took place. It was only when the pace began to slow and the story sagged in great big drifts that I knew I'd punctured the momentum of the story. The sharp point? An earlier chapter mistake that revealed something way too early and knocked the wind out of the story.
Send in the clowns, yo.