Sometimes the littlest things make the biggest difference.
I just had the final artwork returned to me from a forthcoming (September) picture book called
Space Boy, and was reminded of a tiny little change that we made in the main character's facial expression months ago. (When I say "we," I mean myself, the designer, and editor.)
After taking a peaceful moonwalk, my main character Nicholas begins to be reminded of some of the nicer memories of his life back on earth... and in the art I painted him with a little smile on his face. Here is a close-up (the actual illustration is a double-paged spread) of Nicholas in my original piece of art:

He looks pretty happy here, but it really was a more ponderous, reflective moment in the story for our little hero, and so my editor suggested that his expression be changed accordingly. I agreed, but didn't want to have to redo the entire piece of art (lazy!), and through the marvel of modern technology, only painted a few new, tiny, little one-eigth-of-an-inch faces (one to be chosen) to be superimposed over the original piece of art by the amazingly talented designer that I was also working with on the project. Here's what the little faces looked like:

After that designer magic, here is a new close-up of the spread from the final galley of the book:

I think it was worth the effort to make the change; now I'm satisfied that the text and art suit each other in a more unified way in this scene. Yay for thoughtful editors and talented designers!