Continuity mistake: During the alien's assault on the ship, one crewman is seen near the spaceship's stairs firing a weapon. However, the weapon is pointed at a forty-five to sixty degree angle - at this angle, he would hit the bottom of the ship, not the alien that is much farther out.
Factual error: At the end of the movie the cast is counting down to the explosion of the Krell furnaces. They reach zero and the furnaces explode. They see the light from the explosion just it happens. But they are 100 million miles out by then. The light would have taken over 8 minutes, about as long as light from the sun takes to reach earth, to reach the ship.
Other mistake: When the Id creature is melting through the door and Morbius is denying he subconsciously created it, the Commander draws his blaster giving the impression he will kill Morbius to save them if necessary. But Robby deactivated the blaster just a few scenes prior. Was he going to beat Morbius to death with it?
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Revealing mistake: When Robby brings Doc Ostro in the Residence front room to lay him on the couch, you can see a number of the very thin black cables supporting the live actor playing Doc Ostro. Later, when Robby carries the 'body' out, it's quite obviously a mannequin due to the unnatural angle of the head. A dead person's head would sag against the robot's arm - this one is sticking straight out. The Robby suit was not only quite expensive for its day, but very noisy; requiring external power. As Robby is taking Doc Ostro's body out to the vehicle, Adams attempts to figure out the ID mystery. As he confronts Morbius, Robby walks back into the residence. If you look at his right leg, you can see the power cable trailing right behind.
Factual error: The Doctor wonders how the creature can survive three billion electron volts. This is about 0.0000000006 joules. A baseball traveling a 2 miles per hour is one joule. It would take a lot more than three billion electron volts. It would take about 60 billion billion electron volts just to supply one joule. The energy hitting the monster was less than that supplied by running into a gnat.