I went through this phase when I was about 6 where I strongly believed that the person in the mirror was the REAL me and I was just her reflection, and that there was a thing where we got switched. I think I must've read too much Alice in Wonderland.
A childhood belief about mirrors

others believed things like this
- I used to believe that Alice Cooper and Alice from "Alice in Wonderland" were the same person.— Jessica Tinch
- I used to think that everything involving the name Alice had to be Wonderland or Looking Glass related.Anon
- i always thought I was the reflection and your reflection was a personAnon
- i used to believe that when a person moved away, they switched houses with the person that bought their house— alli
- I used to believe that when you cracked a mirror your reflection would come and take over your body!— XxXAbbeyXxX
- I used to believe that my reflection in the mirror was my evil twin that always had bad hair. XDAnon