When I was a kid my father had a file holder on his desk that was like a hybrid between a letter rack and a lazy susan. His secretaries would put things in there he needed to deal with immediately.
The first time I ever heard someone say "I'll put that in my circular file" I formed an immediate association with the filing tool on my father's desk and the phrase. Obviously very important things went in the circular file!
Well into my adult life I'd use that phrase in conversation, not realizing I was implying that I would immediately throw the item I had just been given into the trash.
Boss: Ok, this project is critically important.
Me: I'll put it in my circular file right away!
Boss: .... Uhh ....
A childhood belief about speaking

others believed things like this
- Having seen enough red octagonal STOP signs in my time, I used to look out for green circular GO signs.— Gary
- I used to believe that, if God would die, Santa Claus would be boss.— Judith
- I used to believe that when I used up all the sheets in my desk scratch pad, I would be married.— Barb
- We didn't have a chimney in our house. Father Christmas had to come in through the letter box.— Ginnean
- I used to believed that "approximately" actually means "important".Anon
- I used to think it was the United STEAKS of America and that cows must be really important in the U.S.— Jake