Visible crew/equipment: In the scene where the knights are attacked by the killer rabbit outside the cave, you can see the boom mic at the top of the frame.
Revealing mistake: You can see the threads moving the submarine underwater. (00:04:55)
Continuity mistake: Near the end, Sam Clayton is nearing the finish. The audio leads you to believe that the horse is breathing VERY hard, but when the camera shows the horse, the nostrils are NOT flared as they would be on a horse that has been run to exhaustion (or even down the block). Also, the people filming this scene obviously have no clue about the nature of sweating in horses. They showed "lather" all through the horse's mane. The ONLY places horses will lather while sweating is where something is rubbing, i.e. underneath the saddle, the bridle or breast collar, where the reins rub on the neck or between their back legs.
Revealing mistake: You can see the wires on the handgun pointed at the two extraterrestrial children when the bad guys are staring in shock at the "floating gun."
Revealing mistake: When Frank kills one of the rattlesnakes with a ski-pole in the RV, you can see that he is just stabbing the floor and not the snake.
Continuity mistake: In the scene after the first real battle of two tribes, the winning chieftain is about to chop off the heads of his enemies and is stopped by Sean Connery and Michael Caine. The chief threatens to lop off the head of the Connery character and is hit across the hand with the arrow he is carrying. The arrow breaks and pieces can be seen to fly off. In the next shot of Sean Connery the arrow is back in one piece.
Factual error: When the Chinese are loading crates onto the truck with St Paul's in the background, two much more modern buildings are visible either side of the cathedral - out of keeping with the 1910s/20s setting.
Revealing mistake: When the protagonist jumps two of the evil cowboys that are after his jewels, the stuntman on the right tosses aside the rifle in a quick deliberate move pretending to lose it. (00:10:25)
Factual error: In the bar in Cartagena amongst the goods advertised there's a sale of slaves of color ("Negroes"). However the movie is set in the early XIX century and the sale the poster refers to (listing very specific figures) took place in South Carolina in 1769.
Character mistake: In the fight in the Shaolin temple the Bruce Lee impersonator fights 4 monks who are supposed to attack him simultaneously, and they do, but Miss amazingly obvious openings, to the point of spending long seconds hopping around without attacking. Many martial arts movies feature as Common Mistake the main character facing multiple enemies that fight quite inefficiently, but few are this obvious when it comes to waiting a cue. (00:13:00)
Continuity mistake: When Molly dismounts her horse near the end of the movie the blanket slides off. A second later the blanket appears back on the horse.