That you literally couldn't conceive unless married - catholic upbringing
That if I ate apple seeds a small apple tree would grow inside me and make me constipated - mums fault
I already had curly hair so wasn't fussed about crusts but this one made my sister refuse to eat hers when she was told it by a dinner lady cos she DIDN'T want hair like mine
can't say as I blame her!
That wishing bad things on people would make them come true - parents! Eg "I wish the earth would swallow you up" said in an argument would make it come true
All my grandparents and quite a few aunts and uncles were called by nicknames by me, siblings and cousins. They were...unflattering...some of them yet tolerated and even viewed endearingly within the family but the result was a combination of
I didn't learn some relatives official names until adulthood!
I (and the others of my generation) caused them to be teased/mocked by neighbours etc as we'd just quite openly say "right I'm off in granny knobbly knees (not a real nickname) has my dinner ready" we were quite young when doing this and so had no idea it was unpleasant and as I say the relatives we used these names on had no issue with it. Often came about due to the "babies" not being able to pronounce their proper names plus some duplicated names causing confusion
That Margaret Thatcher and Arthur Scargill were married! 
Think that's my favourite so far!
I can't for the life of me remember the term - possibly meta awareness? - that describes knowing that something exists when you can't see it, but I genuinely believed that people and things only existed when I was there for much longer than is typical as a child/young teen. tbh I still have my doubts on this one, you lot could all be figments of my imagination for all I know 
I was diagnosed with autism in my early 30s and this is apparently a fairly typical trait, but realising that other things existed without my being there weirded me out.
Ohh interesting. I have ocd dx and several mh hcps and my current gp have all said they suspect I may have Aspergers never been tested though I'm 48
I regularly have dreams where I just have a normal day in the dream, go to sleep in the dream then when I wake up I'm not sure if I'm still dreaming or really awake
I also had times I believed I was being watched/recorded at all times that everyone was. Whoever wrote "the Truman show"basically obviously had the very same idea. And the film cams out waaay after I was having those thoughts! So I wonder if they're actually very common?