Here's some more silly things:
I believed that if you sat in the bath til all the water drained away, you'd be sucked down the plughole. So I always got out the tub as soon as possible to stop that happening. XD
I also thought that teachers lived at the school inside the staffroom when I was at primary school. In secondary, I'd discarded this belief but I thought that the staffroom was like a pub where the teachers went to booze and party, and that's why we weren't allowed to go in there. XD I was really disappointed the day a friend and I peeked inside.