In the Christmas song "Joy To The World" I thought the words were: Let every heart prepare Him a room.
It made sense to me because I knew the story where there was no room at the Inn for Joseph and Mary.
A childhood belief about hymns

others believed things like this
- I thought "Virgin Mary" was actually "Verge 'n Mary" and that Verge was a nickname for Joseph. — Rachel
- I thought the song "Party Time" by Moby was actually saying "heart attack" (this was before I knew the title)— Elviessa
- I used to think Joseph of Joseph and his amazing techni-coloured dreamcoat was Jesus' dad. Anon
- I thought that the star that Mary and Joseph followed was called Yonder, as in 'following Yonder star."— Kairow
- I believed that Jesus Christ was the son of the Christ family, i.e., Mary and Joseph Christ.— David Araújo
- The song called heart of glass mixed me up as a kid. I heard "Heart and ass". — oops