Continuity mistake: When Quatermain arrives in London, he is greeted by Reed who leads him down some stairs. In the shot where they're going down some spiral stairs, for some reason you see a different actor (who is clean shaven) posing as Reed, going down the stairs holding the umbrella in his right hand. Then in the next shot you see the "original" actor who has a slight goatee and is holding the umbrella in his left hand.
Continuity mistake: The invisible man is seen in one shot with cream covering the majority of the front of his face but it ends towards the middle of his head. When he is in the automobile, the cream is now completely covering his head and neck. Then, when they go to see Dorian Gray, only his face is covered with make-up again.
Continuity mistake: When the Phantom's men ambush the League at Dorian's residence Sawyer is seen joining them in revealing themselves and aiming at the League (you can tell it's Sawyer by his clothing and weapon). Then during the Phantoms speech Sawyer reveals himself again as if he was never there.
Continuity mistake: When Quatermain realizes that Skinner is in his room (on the Nautilus), he puts out the lamp and the room goes black. We hear a scuffle and then the door opens and lights the room and we see Quatermain throwing the invisible Skinner out the door. But when Quatermain shuts the door, the room does not go dark again.
Continuity mistake: In the scene on deck of the Nautilus where Quatermain and Sawyer practice shooting the target balls are lying in a row next to the railing, first five, then four, but before the last shot, four again, although another target has been catapulted into the water.
Continuity mistake: When The Phantom kicks Quatermain's gun along the floor we see the gun landing close to the wall. In fact the tip of the gun even sticks a bit in to a hole in the wall. When Quatermain a few seconds later picks the gun up the gun has moved at least a feet away from the wall.
Continuity mistake: During the brawl in Kenya at the beginning of the film, one of the goons is seen throwing knives at Alan which seem to pin him to the wooden pillar; yet in in a previous shot just before Alan slams the table onto one of the goons, the knives are already embedded in the pillar.
Continuity mistake: When The Phantom tells Quatermain to drop his gun, Quatermain drops it. It cuts and we see the gun landing with the handle pointing out. The way Quatermain dropped the gun the handle should be pointing in. The gun could not have changed angle that quickly when watched how it is dropped.
Continuity mistake: When Dorian Gray is shot with the machine gun, all the bullets are taken right in the gut, and the wounds are correctly shown in mirror-image when the back view of him is shown. However, the next shot of his front shows a couple of wounds in the shoulder, the biggest one being in his right shoulder at the very top, a wound which was not visible in the back view.