Revealing mistake: On the collapsing bridge, when the Phantom saves the kid, they swing on a rope hanging from the middle of the bridge. From a wide angle, it's very obvious that there is no possible way whatsoever that the short rope reaches the safe edge of the cliff. When the angle changes to the edge of the cliff, the rope is suddenly hanging from somewhere right above the edge.
Revealing mistake: When Diana is about to jump off the hydroplane, the safety vest on her back protrudes. Also, a safety bar along the plane's left side has been added for her to grab.
Revealing mistake: The natives use a pair of nooses to trap the bad guys, but when they're lifted up, one can see the protruding hook and mechanism under their shirts.
Revealing mistake: When the Phantom chases the truck, he holds onto a branch where he later stands to attack the truck. In the close-up, one can see it's an obvious metal bar to allow for a safe stunt.
Revealing mistake: When Diana is about to jump off the plane onto the galloping horse, her hair moves wildly. When the shot changes, she has been replaced by a stunt who looks nothing like her and has her hair fixed.
Chosen answer: The explosions from the skulls and the weapons on the island caused the volcano to erupt.
LorgSkyegon