I literally just had one hit me out of the blue while browsing this site. My thoughts turned to Cheshire Crossing, and I suddenly realized — Oh! The "I can't wait to teach her about isosceles triangles" bit between Dorothy, Scarecrow, and Tik-Tok is a joke on the mistake between isosceles and right in the movie! I'd just skimmed right over it without a second glance before (granted, I was reading the series mainly for Alice).
Just hit me why Rutherford has Lemuel Gulliver as an assistant (although having realised it, it seems like I was just being slow on the uptake); obviously he's the world's foremost expert on travelling between multiple fantasy worlds.
The Wicked Witch mentions the rarity of planar adepts. Now Oz is quite small and Neverland even smaller, so it's normal that she can't fathom how vast the Earth is.