Okay, first childhood misconception I can remember about SW:
In the early days before I'd seen the movie or even much in the way of stills from the film, I remember seeing the cover of the original Star Wars novel (which my uncle bought before the film came out)...
...and I was absolutely certain that Darth Vader was a giant evil robot, bent on eating and/or crushing those little characters down below him. Subsequent movie posters and promotional material featuring a giant Vader (or Vader head) towering over characters in the foreground did little to dispel that theory.
I thought the Stormtroopers were robots as well, of course, when I saw pictures of them. What else would they be?
--SKot