I just thought of what is probably my biggest childhood misconception. Biggest not in plot or story development, but biggest because I STILL have this misconception today!
Probably the most famous line in Star Wars: "No, I am your father," I still to this day hear as "No, I am the father." And it's sooo hard for me to remember that that's not what it is. It's usually after I see ESB that it occurs to me that I heard it wrong again.