I used to believe that the expression "old-timey" was actually "old Tommy." My great uncle's name was Tommy and with my mother's thick southern drawl, "Tommy" and "timey" sounded very similar.
I thought this until I was at least 12 or so.
A childhood belief about speaking

others believed things like this
- I thought that Tommy Lee (the singer) and Tommy Lee Jones(the actor) were the same exact person.— Glowworm
- I thought the expression "so funny I forgot to laugh" was literal instead of sarcastic— Not so funny after all
- I used to think that the expression "The former or the latter?" was actually "The farmer or the ladder?" — Fitzwilliam
- I forgot the expression "not cricket", so I said, "not tennis" instead.Anon
- I used to believe that your soul was a millimeter thick, right under your skin, and looked exactly like youAnon
- My friend told me she had invented the expression 'Gordon Bennett' and I believed her!— SueE