Or perhaps more to the point, what you weren't remembering. Work your way through a knotty passage, and, it turns out, you may not remember what you were writing before it. It was days -- weeks? months? -- for you, after all, and it's only reading through at a canter that you remember it was only twenty pages ago that you harped on that theme before. Does the story structure really work with these two problems bunched together, when they are so similar?
Or you forget and set up a situation two and three times, so that your readers, reading through the book in one merry day, will find that matter thrown into their faces more than is needed. And the same for exposition. One thing that can help with that is making a note of what is needed and then crossing it off once you do it, but that's not entirely reliable.
And other discoveries about inconsistencies, and needed motivations, and planting guns on mantlepieces -- very wise, revising a work at a canter before you pronounce it done.