Lt. Commander Giotto wears full commander stripes.
Several times in this episode, Kirk uses his communicator and it doesn't "chirp" when he opens it.
Vandenberg opens the door to his office with a button on the underside of his desk - at different times he presses different spots on the desk.
Why did the Horta steal the reactor pump rather then destroy it? It's trying to kill the miners or get them off the planet - what advantage is there for the Horta to save it?
How did the Horta know exactly what part to steal? It's smart, but it doesn't seem to be familiar with human technology.
During the confrontation with the Horta with Spock and Kirk, in one shot, Kirk's phaser is up, and the next shot, the phaser is down to his side.
While Kirk is waiting for Spock to arrive, after his meeting the Horta and sitting down, we see him holding his phaser pointed at the ground, near his waist. We then cut to a wide shot when Spock comes in and we see Kirk holding his arm stretched out, only this time he is holding his communicator. Then we cut back to a close-up of Kirk and his arm's holding the phaser again, pointing down.
When Spock and Kirk pursue the Horta, they come to a fork in the tunnel. Kirk points right and tells Spock to go left. Then he points left and says he'll go right.
Spock informs Kirk that the Horta excretes a highly corrosive substance that lingers on the tunnel walls, yet in later scenes a portion of the creature and eventually the creature itself is touched with no ill effects.
The Horta's tunnels don't match the shape of its body - watch the one scene where we actually see it going down one of its tunnel. The Horta is kind of oval/elliptical, but the tunnel is perfectly round and several feet higher than the top of the Horta.
For all their high-tech advances, McCoy uses a plastic yellow bucket to hold the thermo-concrete he uses on the Horta.
Vanderberg: Look, we didn't call you here so you could collect rocks!
McCoy: He's dead, Jim.
Spock: If it is the only survivor of a dead race, to kill it would be a crime against science.
McCoy: [after being asked to treat the Horta] I'm a doctor, not a bricklayer!
Spock: The odds against both of us being killed are 2,228.7 to 1.
Kirk: 2,228.7 to 1? Those are pretty good odds, Mr. Spock.
Spock: And they are of course accurate.
Kirk: Of course. Well, I hate to use the word, but, logically with those kinds of odds you might as well stay. But please stay out of trouble, Mr. Spock.
Spock: That is always my intention, Captain.
Kirk: Think she'll go for it?
Spock: It seems logical, Captain. The Horta has a very logical mind. And after close association with humans, I find that curiously refreshing.
Spock: Curious. What Chief Vanderberg said about the Horta is exactly what the Mother Horta said to me. She found humanoid appearance revolting, but she thought she could get used to it.
McCoy: Oh, she did, did she? Now tell me--did she happen to make any comment about those ears?
Spock: Not specifically. But I did get the distinct impression she found them the most attractive human characteristic of all. I didn't have the heart to tell her that only I have...
Kirk: She really liked those ears?
Spock: Captain, the Horta is a remarkably intelligent and sensitive creature with impeccable taste.
Kirk: Because she approved of you.
Spock: Really, Captain, my modesty...
Kirk: Does not bear close examination, Mr. Spock. I suspect you're becoming more and more human all the time.
Spock: Captain, I see no reason to stand here and be insulted.