Actually, the mistake is the person can't do math. And it isn't isolated.

Have you ever tried to give change after being run up at a Starbucks or something? They owe you 20 cents, and after digging around you find you have a nickel. So you give them a nickel. And they give you back a dime and a nickel.

It is indicative of how the whole world is losing its ability to do math. Even simple calculations require a calculator for most people nowadays.