Lousy memory coming back to haunt me. I was on our HS math team.
We had a problem once, a small piece of which required us to know how many teaspoons in a tablespoon. I had never learned this in math or any other part of my school education. However, being a momma's boy, I was always in the kitchen and had asked my mother who told me that 4 tsp = 1 Tsp. So I missed a sure thing.

Anyways, I learnt the meaning of "trust, but verify."