Revealing mistake: In the scene just before the final fight scene, when Burt Lancaster is in the balloon and is approaching his 18th century sailing ship, the scene gradually changes to a view of the side of the ship. Just then and for a few seconds you can see a white cruise ship on the horizon in the background, just above the deck and to the left of the forward mast.
Vallo: You've sold me Humble Bellows - and to a king's flunky.
Humble Bellows: Aye. 'Tis my modest opinion that no man can fly pirate colors who's not willing to sell his friend, his sweetheart, or his mother.
Baron Jose Gruda: Well spoken, Mr. Bellows.
Vallo: Foul spoken it is, Humble Bellows. You've turned your hand against your captain's back. Yellow was never a pirate's colour.
Slimy: Nor rescuing a fair maiden a pirate caper.
Humble Bellows: Be that bit of fluff El Libre?
Vallo: That's the bit of fluff that's taking us to him.