On the first two novels I wrote, I stuck with the outline pretty well, and the second version wasn't too painful.

This third novel, though, had a derailleur sort of like what you described. I added a POV character about 1/3 of the way through writing (she was there before, just didn't have POV) and it skewed EVERTHING out of place. But I liked that POV, so I stuck with it.

Usualy, it's not this bad!