I used to believe that 'bell end' meant just that. The end of a bell. I had heard my friend call someone it when we were 11 (and female, for the record), and as most calling words were just objects at the time anyway ('spanner', 'spoon', 'plumb' etc) I assumed it was just the object.
Then later, I was in the car with my Dad and some guy cut him up. My Dad shouted "Idiot!" and I pipped up "Yeah, stupid bell-end!" and my Dad screetched on the brakes, said, "WHAT?!?!" and asked me what I had said and if I knew what it was. I said, what I thought and he didn't seem to believe me but drove off and it was never mentioned again. Except a ton of times to me mates in the pub.
A childhood belief about rude bits

others believed things like this
- I used to believe that Alexander Graham Bell was not just one person. I thought that they were three people!— J.D.
- When I first heard the words "flea market", I actually thought it meant "selling fleas".Anon
- I used to believe that if you played a record too many times it would wear it out.— April
- I used to think that to desex an animal meant to turn it from a male to a female or female to male.— Phil
- When i was little for some reason i thought that all people from china knew karate.....yeah.— sneakydean
- I never knew how tampons worked, I always thought they would fit in a female flat, like a hot dog.— only me