One of the kidnapped girls told the pirates before jumping off the seaplane that there are 15 of them. After Porco struck the pirates, there are 15 girls safe in the swim ring but next scene of Porco's seaplane,18 girls are playing around.
Near the end of the film when Porco and Curtiss are fighting 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 its bottom two stripes; most likely it's a simplification for animation.
When Porco sees all those planes that were shot down throughout the First World War, there was one RFC plane called a Martinsyde Elephant. Only one was built and it never entered service or saw action during the war because of its weight and faulty air frame.
The fascist logo on the planes is upside down.
At 54 minutes in, Porco is in a pub with a Shell logo on the outside wall; that logo was used from 1971 to 1995.
Milan don't have any river.
There are spelling mistakes in all three Italian texts that appear in the movie: first newspaper "mamma aito" instead of "mamma aiuto" and "L'bambie" instead of "Le bambine", second newspaper "vivo a morto" instead of "vivo o morto", third sign in Piccolo's office "non si fo credito" instead of "non si fa credito".