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".
One of kidnapped girl talked to 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 there are 18 of girls playing around.
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.