Will loses his right boot and turns invisible. His first set of invisibly-made footprints shows a right bare footprint, but his second set shows a left bare footprint.
When the creature is in its invisibility state, its footprints are shown to be three-toed. However, when the creature materializes, there are more than three toes and each toe has a large claw turned and pointed downward.
After the invisible monster chases the Robinsons into the space ship, the ship loses all power (due to the monster feeding off of it). With the power still out, the thumping outside the space ship stops, the horizon is shown with the planet's sun rise, and the lights on the space ship's control panel are incorrectly shown as on. A moment later, the space ship's control panel lights are shown as off, and then the power comes back on. The control panel lights then light up again.
When Penny and Dr Smith are using the Ouija Board, Dr Smith's fingers change position when the camera zooms in on them.
If you look closely at Penny's right hand during the ouija board scene with Dr. Smith you can see she is wearing a flesh colored glove just before the glass breaks.
During the séance the robot's guitar playing has the sound of an electric guitar, but he is playing an acoustic.
The base of the glass table used for the séance is the scanner antenna of the Jupiter 2.
Dr. Robinson should have read the sign he was planting. As he pounded it into the ground, the sign released a gas which overcame him.
Dr. Robinson carries a warning sign to be planted close to where they are doing their explorations. The only other English-speaking members on the planet are the three women left behind at Jupiter 2, far away from the men. On an alien planet, no one else would be able to read the sign that doctor Robinson planted.