When I was very young, I thought that my Dad was Graeme Garden - they both looked very similar in the mid 1970s. I sort of knew that they weren't actually the same man, but when my Dad went out to work, I assumed that he went to an office just like the one in the Goodies, had adventures and rode about on a trandem with his own Tim and Bill stand-ins. I'd only actually seen the Goodies a couple of times, when I was allowed to stay up late because of illness, but it must have made an impression.
A couple of years later on, when I'd found, much to my disappointment, that he was a primary school teacher, he'd often make references to a mysterious entity called "the Boss", obviously the headmaster. Somehow, I imagined that the Boss was a vampire, who only came out after the school had closed, and stalked the corridors at night. Eventually I sussed out that the Boss's real name was Dicky Lowe, but I was still terrified in top infants when we were told that Mr Lowe would be paying a visit to talk to us about going up to the "big school" next door. Somewhat relieved when he turned out to be, not a vampire, but a stern-looking old man with a certain resemblance to Sam the American Eagle in the Muppet Show.
I also used to think that TV, late at night, showed nothing but nude people and horror films. This was based on (a) whenever I woke up late on, and went into the living room, there always seemed to be some Hammer or Amicus potboiler on the telly, swiftly switched off by apparently guilty parents, and (b) I was told that I couldn't watch Monty Python's Flying Circus, because it featured "A bare man playing an organ." This last one gave me a strange image of Monty Python well into the 70s - I thought it was a real circus, with elephants, clowns etc, but with musical interludes from the bare man!
Just remembered another one, probably brought on by early exposure to the film of Jason and the Argonauts. I thought that skeletons were alive, could walk around, and were a bit nasty, which is why no one ever talked about it. Bones, skulls and the like, seen on news reports or in museums, were actually dead skeletons. I think I went into denial for a while after I found out that we all had skeletons inside us!