I read this wonderful story in the fascinating book Dear Genius: The Letters of Ursula Nordstrom.
Nordstrom was an extraordinarily brilliant and influential editor of children's books, and here, she makes an important point about how sometimes, flawed can be more perfect than perfection. When reading that picture, book children will enjoy noticing that in one illustration, the octopus has only seven tentacles.
It can be a lot of fun to point out mistakes -- something I remind myself when people seem to take such great pleasure in pointing out my mistakes to me.
The beautiful portrait above of Nordstrom was painted by Maurice Sendak, whom she discovered and published.