I used to think that "a couple" meant any small amount, such as 3 or 5. My mom once corrected me and said that "a couple" always meant 2, and that if I meant some other small number that I should say "a few" instead.
I told her that she must have it backwards, and that "a few" must mean 2, because "few" and "two" rhyme.
A childhood belief about counting

others believed things like this
- I used to believe that rounding a number meant drawing a circle around it.Anon
- i used to believe that sex meant love and that having sex meant the two people would live happily ever after.— amy
- that the word "couple" as in "a couple of items" was synonymous with "several", rather than "just two".— Blair A. Petterson
- Someone told me People and fish went to the same place.I thought they meant humans get flushed— MrK
- I used to believe whenever I got an erection it meant I had to pee.— Adam
- I used to think that the E on a gas gauge meant "enough"...Anon