In order for John to reach the room inside the cave containing the possessed alien mask and other artifacts, it was necessary that he be lowered down a chasm using a rope. However, later when Will and again when Don, Maureen and Judy enter the cave this room is now at ground level.
During the fight in the cave between Professor Robinson and Major West, Major West's sword is pulled away by Professor Robinson's weapon. In the next shot it reappears in Major West's hand.
When the alien Canto approaches the Jupiter 2, the robot alerts Will that he is coming. As Canto walks toward them, his shadow can be seen on the artificial background scenery.
When the trying to clear the passage way after the cave in, Will leans on a bolder and it flexes under his weight revealing that it's make of rubber.
When Major West, Maureen and Judy enter the cave and John/Canto says "What is it you want here?" he is clearly seen standing in position before flash to make him materialise to the others.
When fully possessed by Canto, John leaves the cave with a warrior glove, sealing up others behind him. When he thereafter appears at the Jupiter 2, the warrior glove is gone and he's disguised by a mask, which was last seen back inside the cave.