I have lost track of the moon.
Not the actual moon above Rochester, which yesterday was just shy of full and visible briefly before our usual clouds obscured it. The moon I've misplaced is in my YA fantasy, which is now nearing the end of draft #2. Moonlight matters in pre-industrial societies, because it and that stars provide all the light there is for night traveling, which my characters are doing a lot over rough terrain. I need that moon, and I need it to be full at certain times. It also means I can't have, say, two full moons within two weeks, and so time passing in the book has to be coordinated with phases of the moon, and I've lost track.
This is the kind of thing a third draft is supposed to correct. But because I've tied night traveling so critically to my plot, I need to fix it now or change plot events. So I am drawing charts and schedules of the moon for this non-existent country ("The Queendom"), and trying to remember where in the ms. I said there was a tiny crescent moon high in the sky and will that fit with this later development?
And I still have money to do. How big a store of precious coins did Roger have, how much has he spent so far? And how much does a cup of sour ale cost?
Science fiction, of course, has other versions of this detail-fixing (How long can that gizmo's battery last? What's it powered by? Does that neurotransmitter really do what I just said it does?) It's niggling and necessary and produces reams of notes on legal pads. I am drowning in paper right now. And the moon is a waning crescent.