How can you forget? Growing up in Australia, every computer I’ve ever owned has defaulted to “US Letter” as the paper size. (Even when your computer was set to use Australian localisations.) If you didn’t change the setting, you couldn’t print. The printer would complain that it was out of paper, because it didn’t have a tray with the requested us letter paper in it. It was one of those things you always had to check.
I don’t think I saw a “us letter” sized piece of paper until I was an adult.