I used to believe "certain" (used as a verb) was an uber-secret religious ritual performed on reluctant shepherds. I knew this because it said so in the song. "The First Noel the Angels did say, was to certain poor shepherds in field where they lay."
A childhood belief about hymns

others believed things like this
- I always wondered why shepherds washed their socks by night. (correct lyric: watched their flocks)— MaggieNJ
- In a religious song, instead of “Jesus Christ” I heard “Cheese is Prize”— EJ
- I used to believe that the TV could see me back. Noel Edmunds told me so.— Ringo
- i really thought that "key" was a secret letter to the alphabet that only i knew.— amber
- When I was young,I thought the lyrics "Secret Agent man" were actually "Secret Asian Man".— M.M.B.
- I believed Noel Edmonds real name was 'Mole' Edmonds— abi