Other mistake: An antenna can be seen on the building above the grain warehouse.
Other mistake: When Bellow is pulled up from the back of the ship after eavesdropping on Lancaster he is soaking wet, yet he was not lowered into the water.
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.
Other mistake: The girl cuts the pirate's hand when he asks to see her knife.
Revealing mistake: In the last scene with the balloon approaching the ship, one of the trailing lines from the balloon arcs upwards and off screen left instead of trailing straight down. Most likely the balloon was on a crane and the line is a guide line. (01:33:00)