That sounds like a pretty hard and fast rule, but it's something that became glaringly obvious to me during revisions of Ondine 4.
Yes, I'm still revising Ondine 4. It will be ready when it's ready.
It was too loose and took far too long to get going. And then it hit me. I had created a whole new 'problem' for Ondine to solve, which was not really related to her existing issues. Nor was it a true complication of an earlier problem. It was a plot thread that would not fit in.
OK, it sort of was, but as that problem was something only mentioned in book 1, it was a problem most people wouldn't even remember.
Resurrecting that old problem into book 4, which in structure terms is the final act of the four novels, meant I was bringing in a whole new complication far, far too late in the piece.
If you think of a novel in the '3 Act' structure, the midpoint is about half way through the second act (hence its name) and from then on, there's no turning back. For example, in Gone With The Wind, there's a really good reason why they put the intermission after Scarlett comes back to Tara and declares she'll never go hungry again. This is the true mid point. Scarlett can't go 'back', the past is gone. She can only move forward, and if she has to lie or cheat or kill, yes, even if she has to kill, she'll never be hungry again.
I love that scene SO HARD!
All the problems Scarlett faces after this incredible mid point are not new. They are further complications that have continued to fester from the beginning.
I love it when my brain does me a solid.
Happy writing everyone.