For a year or two the competition between Luke and Han over Leia went completely over my head. For a couple years after that I still didn't see it in ANH, though I noticed it in ESB. I thought the "I care" scene was nonsense and I couldn't figure it out until I was about eleven or twelve. :P

I think plot points went over my head more than I held actual misconceptions. For instance, I didn't get how the Death Star found the Rebel base since I didn't get the bit about the homing beacon.

Another actual misconception I had was again in ESB. I thought Lando didn't know about the Imperials on Bespin, so I thought he was more of a good guy than he really was and I didn't understand why Chewie choked him.