In Platinum and HeartGold & SoulSilver, Bidoof's gender difference has more to it than in any other game. In Diamond & Pearl and every future game after HG/SS, Bidoof's gender difference is that the male has an additional 2 curls on the tail. However in these 3 games, Bidoof's patch of white around the face has a more pronounced bump in between the eyes on the male, with a more flattened bump on the female. Whether this is intentional or not is unknown, but due to the fact it was fixed later in Generation V, it's safe to assume this was a mistake.