I love that moment where you've been working on a novel for months, you've written two thirds of the draft, and you suddenly get a flash of inspiration which allows you to greatly improve a sub-par subplot. Not only that, you also manage to tie it firmly to the main plot AND foreshadow upcoming events.
This happens to me with every book I write, which is why I'm happy to write chapters and scenes even if the plot isn't quite there. A plot outline should be organic, like a nice green vine, and every author should pack a pair of shears and grafting tape.
Words are free, and you can write as many of them as you want without paying taxes, so bash those scenes out in volume and you never know where you'll end up.
The biggest mistake is to not start writing until the plot is perfect, or to stop writing because you're not sure what should happen next. Write it anyway!