When I was in grade school my mom made my lunch every day, and gave me a nickel to buy milk. One day she told me to buy my lunch at school. She gave me a quarter, which is how much the lunch cost, and told me not to buy the milk first, that I would get a milk when I bought the lunch. So, at lunchtime, I went in the cafeteria and bought a milk, since that's what I knew how to do. Then I tried to buy the lunch, but I didn't have enough money. I panicked, and started crying. Grown-up people tried to help me. Eventually someone figured out what I did.
Unfortunately some variation of this scenario still occurs from time to time. Usually without the crying.