During the opening sequence Tony asks the Captain if he would like to make a deal. Beal steps around the right side of the table to confront Tony. The next shot shows Doug and Tony standing side by side when Doug should be off to the left hand side of the table.
When the British ship is bombarding the island, each cannon ball would explode upon impact. This is impossible since cannon balls have no explosive substance within them.
At one point a runaway ship runs into the island. You can see the island move in response to the ships impact, revealing that the area stuck by the ship is a prop.
The Barbary Pirates were based on the coast of North Africa coast in modern day Libya. The pirates were for the most part Arab Muslims. The pirates depicted here are European and dress as if it's 1705, not 1805.
This has an inexcusable mistake concerning the island whereof the forest expose in the story wasn't accurate due such place is a dry territory, so put a lavish rain forest in such area is a fateful error made by Irwin Allen's crew,
Right at the beginning, as Tony and Doug materialize and drop to the deck of the pirate ship, if you watch their shadows, you'll also see the shadows of the ropes they were holding to allow them to float through the air.