When Kirk and Spock shoot the Horta with their phasers, nothing appears to happen, and the Horta backs off into a tunnel it creates. Kirk and Spock run over to the new tunnel, across a floor that is clean. When the shot shifts to show them from the tunnel, there is a piece of the Horta on the floor (to the left) where there was none before (in the previous shot). When the view shifts back to showing the chamber they are in, the piece of the Horta is on the floor.
When Kirk and the Horta have squared off against each other, and Spock enters, there's a two-angle back-and-forth. When the camera is directly at Kirk, he is kneeling with a phaser in his right hand. When the angle switches to Kirk at left, Spock far center, Kirk is kneeling but with a communicator in his left hand. The change goes back and forth a couple of times.
Even though it takes place at one time, Kirk wears two different uniforms (the older one has tighter braiding on the sleeves and has shrunk considerably).
Spock says he's surprised that the "roof" over Kirk's head in a cave had collapsed. Since the caves are miles underground, one might think that Spock would have likely used the word 'ceiling', but a check of any dictionary will reveal that "roof" is a perfectly acceptable term for the top of a cavern, cave or enclosed passageway.
When Kirk is cornered by the Horta, a shot from behind shows that it is a stunt double leveling his phaser at the monster.
After Kirk and Spock fire at the Horta, it gets away through a tunnel, and, after Kirk mentions that the walls are hot, Spock notes that the tunnel was cut "within the last two minutes". However, when the Horta was fleeing, the tunnel clearly already existed.
When Kirk and Spock shoot the creature after seeing it for the first time, it retreats into a tunnel. The tunnel has a flat bottom as it moves away but in the next shot from the reverse angle it has a rounded bottom.
When in the Horta cave after Spock's mind meld, Kirk is holding a broken egg shell. The molding seam is visible on the prop.
In the prologue scene, Schmitter remarks that at least 50 have been killed, and adds Appel shot it and look what happened to him. But Appel survived and gave dialogue in subsequent scenes.
When Spock first approaches the Horta for a mind link, in the background where the words "NO KILL I" would soon appear, a rectangular covering hides those words as they are not yet ready to be seen in the story line. The edge of the covering can clearly be seen at this time.
In the end credits, George A. Rutter is listed as "Scpipt Supervisor".
Using clubs to attack the Horta would make no sense. Since the Horta exudes an extremely corrosive fluid to create its tunnels, capable of dissolving large quantities of solid rock in seconds, all it would need to do to protect against the clubs is release a large quantity of the corrosive. Any kind of club used to attack the Horta at that point would simply disintegrate when it touched the corrosive on the Horta's exterior.
After Spock's initial mind-meld with the Horta, Kirk and Spock discuss asking the Horta where the 'Retardation mechanism' (for the main reactor) is. There is never any indication that they knew the mechanism was cut out and not simply dissolved away by the Horta. Nor is there any logical reason why the Horta would keep and not just destroy the reactor pump, if it wished to drive them off the planet.
Spock warns Kirk about touching the hole in the reactor room door just cut by the creature. But just after he and Kirk shoot the Horta, removing a piece of it, Spock unhesitatingly picks it up.
When deciding which of two tunnels to investigate, Kirk tells Spock to take the one on the left, and he'll take the one on the right. But he gestures with his phaser to the exact opposite tunnels.
In his line "Silicon-based life is physiologically impossible," Kelley says "silicone."
Kirk tells Spock he's 10 feet away from the monster, rather than counting in metric system.
Early on, Kirk asks the head miner if he placed any sentries, but he pronounces it "centuries."