This isn't technically UNDER my bed, but...for quite a few years, I believed that if I didn't have my arms under the bedcovers, the KGB would break into the house and cut my arms off. (Why the KGB? My family listened to lots of religious radio shows, and one of them was about little Soviet children and their families who were being persecuted for their faith. Thanks a lot.)
A childhood belief about under my bed

others believed things like this
- I used to think that you would get arrested if you didn't swing your arms when you walked in publicAnon
- We used to think that if you ran swinging your arms around, you would run faster. — Christine
- that if I flapped my arms hard enough, I could get airborne.— DeQuendre
- When my grandma was little, her dad would tell her that the freckles on her face and arms were fly crap.Anon
- I used to think that 'testicles' was the word for the arms of a squid or an octopus. — Emily
- I could never work out how they used to change the bands over so quickly when i listened to the radio— graham