Summertime, but the weather is freezing

Summer vacation is officially upon us! And I'm wearing a sweatshirt.

Unfortunately, someone forgot to inform Mother Nature that it is summer, not spring, vacation. Temps barely reached 60 degrees for this first day of swim lessons. Just about half the kids in my son's swim class cried at some point during the hour. One kicked the instructor when she tried to pull him into the pool. My boy glared at me from start to finish.

Don't you hate it when things don't work out the way they're supposed to?

I've got the same issue with the manuscript I'm writing now. I've got to figure out how to add a scene to a section that flows perfectly without it. But if I don't somehow weave this scene in, the entire plot crumbles.

I've read books before where I can tell the author had the same issue. Suddenly there is a new character or development that instead of being a twist in the story, it's an absolute yank from it.

On the other hand, I've read books where the development is so subtle and natural that I totally miss it. It isn't until my second or even third reading of a book that I pick up on the artfully placed hints. I think J.K. Rowling is an absolute master of this. A character I might first dismiss as comedic relief (Gilderoy Lockhart comes to mind) plays a pivotal role several books later.

Which authors do you think are great at making readers feel the action is unfolding just as it should?

Writers, how do you handle adding scenes without a total rewrite?