Biggest mistake I ever made was when I was in, maybe, 4th grade.
My mom was always wiling to let me tackle cooking and baking unassisted. One afternoon, I decided to make peanut butter cookies for the family. After the first batch came out of the oven, of course we had to sample a cookie, and found that it tasted horrible! I went over the recipe carefully to discover my mistake and found I had forgotten to add the peanut butter! Though we could not eat them, our free range chickens loved them!!