Continuity: In the scene with Mabel and Frederic stripping before swimming out to the pirates' ship, Frederic is seen taking his shirt completely off. But in the following shot, his shirt is still half-way on.
Anachronisms: The film begins with Frederic celebrating his 21st birthday, later dating his actual birthday as February 29, 1856, which would make the present year 1877. However, Mabel later remarks that they were living in the 1880s.
Revealing mistakes: When the Pirate King has his sword at Frederick's throat at the end of the library fight, the safety button on the tip is visible.
Continuity: After Frederick shuts the Pirate King in the coffin and runs swords through it, the swords completely disappear from the coffin when Frederic lies down in it.