My kid's already got a Star Wars misconception. He's only seen Star Wars so far (so far he's been able to wait for the Despecialized 2.x series), but I call this one the Darth Vader/Pootie Tang crossover misconception.

Basically Darth Vader is occasionally using big unfamiliar words for a kid (ambassador, etc), and he's wearing a mask which kinda muffles everything he says. Then practically the first thing Leia says to him is that she has no idea what he's talking about.

...so, later, my kid just starts laughing in the middle of the Death Star conference scene, and says, "Ha! Nobody can even understand what [Vader's] saying!"

Which I think is great! It points to a whole new source of motivation for Vader. The sheer rage of being misunderstood wherever he goes. Finally at the conference he just snaps and force-chokes a guy who misunderstood what he said. I think it's great. Also a fairly decent motivator for Bane in The Dark Knight Rises, now that I think of it.

EDIT: I should also add that my son is young enough that he's got at least three friends who are pretty hard to understand because they're still having trouble pronouncing certain sounds, so a guy just going through life not being understood by the people around him is actually not that out of the ordinary to him.