Continuity mistake: When Kirk, Bones and Spock are commanded to stop, and raise their hands, they do so. After a moment, Kirk starts to lower his hands. After the camera cuts to a shot from behind, Kirk's hands are by his sides and he goes for his phaser. After another warning shot, he says "I think he means it." The camera goes back to facing them, and his hands are up again.
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Revealing mistake: When the "pleasant" Kirk and Spock go down to the engineering level to get the "animal" Kirk, they split up and "pleasant" Kirk meets the "animal" face to face. As he steps out from behind a screen type wall, you can see that the "animal" Kirk is really Shatner's stunt double helping to fill the double role. This can also be told by the size difference when the two are close together and the shot is from behind the double.
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Continuity mistake: After extinguishing the fire, Spock pulls a large vacuum tube out of the primitive computer power supply he's constructed on the bed. When the shot changes, the tube is back in place again.
Continuity mistake: After Gary turns on the typewriter and tells Roberta to give her report but not with her fingers, she responds, "Well, how do you expect me to type, with my nose?" But the typewriter types, "How do you expect me to type, with my nose?" omitting the "Well." It didn't edit her question, because we see from the rest of the scene that it types absolutely every word she says until Gary turns it off.
Continuity mistake: Kirk is talking to Scotty on his monitor, after they have finished talking Kirk goes to switch off the monitor by pressing the button just below it.If you watch very closely you will notice Mr Scotts image fades from the screen before Kirk even touches the button.
Plot hole: Spock has absolutely no way to know, yet, that the horta only secretes her corrosive substance when tunneling: he hasn't had time to examine her or to do more than determine that she does indeed secrete a substance that cuts the tunnels. So he should at the very least scan the piece of her that falls off before he picks it up with his bare hands. Major lapse of logic, which given his character goes beyond a character mistake.
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