The Enterprise Incident - S3-E2
Other mistake: When Chapel comes into Sickbay and looks at the "dead" Captain, the camera goes to close up of Kirk, and he wakes up. After she yells for McCoy, the camera goes back to Kirk, and he goes back to sleep. The motions of him waking are obviously played in a slow reverse instead of him quickly closing his eyes.
The Enterprise Incident - S3-E2
Other mistake: As the Romulan commander goes to answer the intercom, the switch she touches to open the link is already on, probably left that way from a previous take.
And the Children Shall Lead - S3-E4
Other mistake: At the end of the episode, Kirk commands the Enterprise to Star Base 4. Earlier in the episode, there was an unsuccessful attempt to beam up two crewmen left on the planet Triacus. They evidently remain there to this day.
Is There in Truth No Beauty? - S3-E5
Other mistake: In the final scene, Kirk is still in the transporter room, but does not have on a visor when the ambassador transports out. Even if he did, humans can still go mad if wearing one and that's why Spock operates the transporter. Spock has his visor on, but then you see Kirk walk out of the transporter room without a visor and he is not going mad.
Other mistake: As Kirk and party duel against the Klingons, there's a heavy-looking round conduit on the wall right before the turbolift door that gets knocked down so it stands as a big flower pot. A moment later, it's back in position.
Other mistake: After the shot of McCoy checking Sickbay, there's a shot of Chekov. It was a quick shot but it can be seen that the film was run back and forth through about a second, as his movements can be seen to repeat rapidly in reverse and forward twice. (00:06:56)
Other mistake: After Chekov goes beserk on the bridge and Dr McCoy and Scotty restrain him, Spock enters the scene and grabs Chekov by the sides of his face. His hands are close together as a human's face is around eight inches wide. When the camera cuts to Chekov's perspective view of Spock, Spock's arms are spread way far apart. Either Chekov's face has ballooned to over two feet wide or Spock is grasping the Panavision camera by its two sides. (00:14:34)
Other mistake: At the end, Kirk teases Spock twice specifically about letting himself be hit on the head in order to determine which Kirk was genuine. But the phony Kirk never hit Spock on the head. He merely pushed Spock over and attacked the real Kirk. Clearly, the action sequence wound up differing from the scripted dialogue, and no one noticed. (Also clearly, Kirk is not speaking metaphorically here.) (00:45:30 - 00:49:20)
Other mistake: The only way to beam up to the ship is to repeat the chess counter move after ASKING to be beamed aboard. After the two Captain Kirks fight ends with Spock stunning Garth with the phaser, Spock contacts the Enterprise. After saying "Spock to Enterprise, and hearing Scotty say back to him "Scott here", Scotty says" Queen to Queen's level 3" without Spock requesting to be beamed aboard or even waiting to see what Spock wanted in the first place. (00:46:56)
Let That Be Your Last Battlefield - S3-E15
Other mistake: At the end of the episode, as Lokai is running through the ship, his hands are empty. However, in one closeup shot he is holding onto a rope with his left hand. The rope is gone in the following shot. Presumably this rope was used to tether him to the camera as he ran so that he could maintain distance from the camera.
Let That Be Your Last Battlefield - S3-E15
Other mistake: When preparing to go around again, Sulu and Checkov start working their controls. When there is a close up of Sulu's hand, it's obviously a re-used shot of Kirk's hand starting the 30 second count down timer on the Constellation in "The Doomsday Machine" (S2 E6).
Other mistake: In this episode, when Kirk's phaser was on overload, it made a small explosion, but in "The Conscience of the King", a phaser on overload would destroy 2-3 decks of the Enterprise.
Other mistake: After Kirk makes the cairn of rocks to bury D'Amato, he places a rock with the man's name on it. If the phasers couldn't cut the rocks of the planet, how did he cut a squared rectangular rock, and how did he put D'Amato's name on it?
Other mistake: Scotty has a science patch on instead of the engineering patch in this episode.
Requiem for Methuselah - S3-E19
Other mistake: As the miniaturized Enterprise appears on the table, the support for the model appears as well.
Requiem for Methuselah - S3-E19
Other mistake: If the ship had not been in visual communication, there's no way Kirk could have seen the Bridge or the crew in suspension.
Other mistake: The time that Kirk winds up in doesn't look anything like the what he was viewing on the disk.
Suggested correction: The entire planet isn't going to look exactly like what he saw on the disk. The disk will show random scenes to give the viewer an idea, but we can't expect the whole planet to match that image.
Other mistake: As Kirk nears the portal, the edge shimmers just before he gets to it. As he goes through it, his actions are greatly slowed rather than natural.