Blooper

During the kissing lesson, Jerry goes from embracing Altaira to holding her by the arms between shots.

The sudden appearance of the cook behind the officers after Robby the Robot first arrives at the ship and his subsequent position relative to them before they leave with Robby.

As Altaira talks to the commander after being found kissing Lieutenant Farman, her position changes from being side-on to face-on to the commander.

When the commander and doctor make their second visit to Morbius' house, and after Altaira's swimming scene, the commander says, "Doc, something new has been added." indicating an action stage right. Doc looks in the direction and agrees, but the audience never sees just what they're looking at.

Robby the Robot shows up carrying lead shielding, which he says is "isotope 217", and he implies it is lighter than ordinary lead. The purported isotope lead-217 does not exist, and if it did, it would be 4% heavier than the most common (and heaviest) isotope of lead (atomic weight 208), not lighter.

Morbius shows the Captain and Doc Ostrow the power units, admonishing them to look only in the mirror. "Man does not behold the face of the Gorgon and live!" They then face away from the real window and look in the mirror. The viewer could believe they are seeing a perspective which is hiding a wall or other shield that they are standing in front of. However, the radiation from the real power unit window completely illuminates the Captain and Morbius's backs. If they are being lit up by the radiation, and it is truly that deadly, it makes no difference if they're looking at it or not. They all ought to be incinerated.

As the Id monster continues to fight its way in to the lab, (with nearly limitless power consumption reaching about 80% of a nearly infinite number), and is turning the 27" Krell metal doors to molten hot, red hot, then white hot, the occupants/humans - due to proximity - would be experiencing extreme, painful or at least uncomfortable radiant heat. Yet they are shown to be unaffected.

A large moon at close range is seen in the sky of Altair IV. An orbiting object of that size and proximity would likely cause gravitational induced seismic disturbances on the planet which are absent from the film.

Altair is twice the size of the sun in every dimension. At the indicated distance from the destruction of the planet, the star should be much larger on the screen than the sun would appear at that distance, not smaller. No zoom out of the viewer was indicated in the script.

The monster leaves a flat footprint, yet the cast of the footprint shows a huge curving claw and other three dimensional structures that could not possibly have been gleaned from the outline of a flat footprint. But the impression of the curved claw is indeed visible, in the front of each footprint.

Dr. Morbius invites Commander Adams to try his blaster on the Krell metal door. Cmdr. Adams inspects the result after firing, but he's not touching the spot where the beam hit the door. He was likely testing near the spot for any warmth as it could be dangerously hot.

There is nothing wrong with the ship landing around 83 degrees north latitude in the "deep Arctic" of the planet. This might be the habitable zone of the planet depending on its distance to its sun, it's angle of rotation, its geography. For example, the only place "habitable" on our own Mercury or Venue might be the "deep Arctic" zones of the planets.

Morbius declares that his IQ is 183. A moment later he claims that the Krell technology doubled his IQ after he discovered it. This would have given him an IQ of 91.5 when he landed on the planet. 91.5 is classified as Borderline Dull. The 183 IQ, then must be a reference to his IQ before using the machine.

When the commander and doctor make their second visit to Morbius' house, and after Altaira's swimming scene, the commander says, "Doc, something new has been added." indicating an action stage right. Doc looks in the direction and agrees, but the audience never sees just what they're looking at. The commander was looking in the general direction of where the doctor knew Altaira must be, so he understood the commander perfectly even if Altaira hadn't been visible to him personally either.

When the ship's cook is picked up by the magnetic crane, one of the wires that really picks him up is clearly visible.

Altaira is supposed to be naked in the water, but closer inspection reveals her to be wearing some sort of flesh-colored dress. This is most noticeable when she gets out of the water and was obviously done to preserve the actress's modesty.

When the invisible Id monster is walking toward the spaceship's steps, its footprints in the dirt are obviously being made by foot-shaped templates beneath the dirt being dropped three inches downward, resulting in the dirt simply dropping down loosely, rather than being compressed by invisible feet.

When Robby takes Doc's lifeless body to the tractor, we see Robby is really carrying a dummy.

In several wide shots of the crew members outside the ship, there is a faint vertical shadow visible on the painted backdrop behind the ship. It appears to be either a fold in a piece of fabric or the corner of the sound stage wall.

Just before they land, Adams impatiently flips off the radio as Morbius is in the middle of the word "recommend". Morbius should have been cut off immediately and in mid-word. But we hear him say the full word "recommend" even after Adams has shut the radio off.

As the vehicle driven by the robot first drives up to the residence, there is a tow cable visible in the foreground, partially obscured by the bushes. This cable is most visible behind the vehicle as it leaves the scene.

When Robby the Robot uses the blaster on the plant in the garden, an unidentifiable shadow suddenly appears on the green V support directly behind Commander Adams.

When Commander Adams leaves Lt. Ostrow to go outside the living quarters to see Altaira in the swimming pool, we hear a film crew member cough.

When the Monster is burning/melting the metal door, as it turns white hot and lumps start to fall, you can see a member of the crew in a heatproof suit prodding the material from behind to make it fall. Watch the second and third holes for movement behind the door. Then again, this motion could be taken to be the monster doing the poking...

During the landing scene, the shadow of the ship (or a supporting structure of the suspended model) is visible briefly moving across one of the smaller mountains at the right side of the screen.

When Adams and Ostrow are about to leave for the Krell lab following the attack on the camp, Adams tells the Bosun to take off the minute the electronic fence on the perimeter shorts out again. But when they get in the tractor and drive away, no one turns off the power to the fence, which means the tractor should have disintegrated the moment it reached the perimeter.

If the size of the input headset of the "plastic educator" along with the shape and size of the doorways/archways suggests the size of the Krell as being considerably larger than the average human, there would be no way that even one Krell technician could fit in the cramped cabin of the shuttle that Dr. Morbius uses to show the captain and the doctor the inner workings of the subterranean power station beneath the planet's surface.

Moebius says the Krell visited Earth before humans evolved, and brought back biological specimens such as the deer and the tiger. But the deer and the tiger are modern species. The Krell would have brought back their distant ancestors. Even if they had evolved, it would not be to the same species that exist today.

Commander Adams complies with Morbius' request to set the self destruct mechanism without first being told the reason for turning a disc and then throwing a switch, not to mention being told afterward it could not be stopped.

The Krell have set up a very easily accessible planet detonation switch. Which if activated by accident, cannot be aborted once the process is in effect. It could be a fail safe that in the event of their extinction. Any alien coming to their world to loot and raid it in their absence, would unknowingly trigger this fail safe and safe-guard the Krell secrets.

When Doc describes their C-57D star ship's weapon capacity as 3 billion electron volts, that value is actually quite minuscule, something less than the energy expended by a flying mosquito.

For example, a weapon like a 20 kiloton nuclear detonation would be on the order of 10 to the 32 exponent electron volts.

While the three men are standing on the walkway in the Krell machine, Morbius points to his right and says "Twenty miles". He then points to his left and says "Twenty miles". Since the three would then seem to be in the center of the machine, that would make it forty miles in total width, yet in the second scene following, he states "Yes, a single machine. A cube twenty miles on a side.". If it is indeed a cube, as Morbius indicates, the Krell machine shrunk from 64,000 cubic miles to 8,000 cubic miles in a little less than half a minute. That's quite an error for a scientist with an expanded mental capacity to make.

Commander Adams tells Altaira that she can't go around dressed like that (meaning her short mini-skirt) so she has Robby make her a new dress where nothing shows "below, above or through". However, the dress she is wearing exposes almost all of her arms, and much of her legs. And after wearing it once for the Commander, she is shown twice later wearing different short mini-skirts again.

When in orbit around Altair IV, Lt Farman is remarked that there are no cities, dams or other large-scale structures visible indicative of civilisation. However, the mission is to investigate what happened to the Bellerophon, which is implied to be the only previous craft to reach the planet. The crew of a single ship would not have produced anything on the scale of cities or dams.

Altaira is referred to as Alta by both Commander Adams and Lt. Farman. That could have been a "pet" nickname (Alta shortened from Altaira) however when she "beams" for Robby to tell him she wants him to make her a new dress even it calls her "Miss Alta" and a robot shouldn't do that unless it was programmed to.