The thing I always run into is that when I start writing I'm generally in a certain "head-space" that tends to mirror much of the plot in some way. After all, writing is very much just bleeding your thoughts/feelings/self onto paper.

But as life happens, I change. At least, in the sense of that head-space... and my jeans once or twice a week. But the plot needs to NOT change. Not drastically anyway. A little movement or whatever is ok, however rewriting or taking some major turn seems wrong.