I used to think the line in "Silent Night, Holy Night" that goes "...holy infant, so tender and mild" actually went "...holy imbecile, tender and mild." Now, I was a smart girl, and I knew what imbecile meant, but I thought maybe it had some esoteric meaning I wasn't aware of.
A childhood belief about hymns

others believed things like this
- I used to think the song"jesus, oh what a wonderful child so holy, meek and mild" was "so holy, Mickey Mouse"— Stephani
- I used to think that an imbecile was some type of seal, before knowing that it meant someone stupid.Anon
- I thought that after the resurrection Jesus became the Holy Ghost.— Meowpossum
- I thought when someone was described as being "mild-mannered" it meant they didn't have very good manners— Samilton Duerheiser
- I used to believe that the Holy Trinity was Dad, God and Santa Claus!— Cathy P.
- I used to believe brussel sprouts were holy because they had a cross marked in the top when mum boiled them!— Ali Brill