I only found out about two months ago that Draco's second name from Harry Potter was Malfoy...and not Malfboy. Childhood, ruined ):
I remember playing Pokemon Red/Blue as a kid, and when the super nerd in Mt Moon says "I found these fossils, they're both mine!", I thought he meant that they were mines, and going to blow up and I shouted at the gameboy "NOW IS NOT THE TIME FOR BATTLING WE NEED TO GET OUT OF HERE!". I also thought Oak's parcel was Oak's pencil...wondered why I had to deliver a pencil...haha.
Another video related misconception! I had the original Sonic games on Sega Saturn and read the manual, and saw that Tails' name was "Myles 'Tails' Prower" or something, and read it as 'prowler'. So I thought he was on the prowl to stop Myles, who I concluded was Eggman. So for years I called him Myles.
I also thought if I put the sonic game in the CD player it would play out songs for each character, and in my mind, the songs went "My name is Sonic, Sonic, my name is Sonic, Sonic, my name is...SONIC" - To the theme of "I like to move it, move it". It was the same for every character q:
Ummm, I remember thinking if I drew over the same pen line hundreds of times, I'd have a wall of ink coming out of the paper haha. I can't think of any more right now.