Near the end of the film when Porco and Curtiss are fight in their planes they crash into a tower, which sends up a variety of international flags. The United States of America flag that appears during this sequence has only 11 stripes and 20 stars, a configuration that never existed. (There were 20 stars on the flag in 1818-1819, but 13 stripes. It's unlikely this is mean to be a rare, nearly 100-year-old flag that has lost it's bottom two stripes.) Most likely it is a simplification for animation.