A Piece of the Action - S2-E17
Continuity mistake: While Kirk and McCoy attack the other two henchmen, Spock shoulder-pinches the card-playing gangster. In the very next shot, Spock's position has jumped from behind the now-unconscious man to several feet away, where he is bending over to retrieve the guns from the floor. (00:16:30)
The Gamesters of Triskelion - S2-E16
Continuity mistake: At the end, the freed thralls, including Kirk, Chekov and Uhura, remove their collars and throw them onto the game platform. But when Kirk and his crew beam away, the collars have all disappeared. (00:48:40)
Continuity mistake: McCoy uses a "masking device" to block out the bridge personnel's heartbeats. When McCoy masks Spock's heartbeat, he holds the microphone, err, masking device over the wrong part of Spock's anatomy. We'd already established that Spock's heart wasn't in the same place as a human.
Continuity mistake: When Miramanee and her companion first see Kirk at the obelisk, the two women are standing, holding their fruit baskets, just inches from each other. In the very next, reverse angle shot, they're suddenly several feet apart. (00:09:10)
A Taste of Armageddon - S1-E24
Factual error: How do the Eminiarians fire a weapon based on sound ("Decibels - 18 to the 12th power") up into the vacuum of space at the Enterprise?
Revealing mistake: For the final time in "Star Trek's" 3-season run, Chekov mysteriously vanishes from the bridge to be replaced by that tall blond doppelganger, thanks to an old piece of 1st season stock footage used for the special effects insertion of the view screen. The costumes are also noticeably different from 3rd season's uniforms. (00:14:05)
For the World is Hollow and I Have Touched the Sky - S3-E8
Continuity mistake: Natira orders the landing party to kneel before the oracle. Spock does so about a foot-and-a-half to McCoy's left. As the shots change, Spock "jumps" to McCoy's immediate side, then back to his original position, etc. (00:10:35)
A Piece of the Action - S2-E17
Revealing mistake: Kirk has just tripped both of Krako's goons with the radio wire strung across the door. He knocks them both out, grabs a gun and then races out the door - right through the space that should still be strung with the trip wire. (00:25:15)
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.
The City on the Edge of Forever - S1-E29
Continuity mistake: While he and Kirk discuss the history tapes in their room, Spock's shirt collar is turned up on the left side in close-ups, but straightened out in the full shots. (00:32:55)
Deliberate mistake: Here, as in nearly every other ST episode featuring caves, caverns or mines, all the floors are perfectly flat and the lighting adequate-to-see-by or even brilliant. These configurations are easily found on soundstage sets - but not in nature. (00:16:30)
For the World is Hollow and I Have Touched the Sky - S3-E8
Continuity mistake: Kirk orders Sulu to match speed with the asteroid, but in the next external shot the asteroid is closing in on the ship.
Revealing mistake: The Horta's round tunnels have perfectly even striations and are obviously factory-manufactured tubes. Acid secreted by an elliptical creature burning through solid rock would not create a perfect circle. The Horta is visibly not chewing, sculpting or smoothing the sides. No acid burning method would leave patterned stripes on the walls, either. (00:18:15)
The Return of the Archons - S1-E22
Continuity mistake: The U.S.S. Archon wasn't violating the prime directive, the prime directive was formed in 2253.
Mudd's Women - S1-E7
Continuity mistake: Kirk's says that it is Stardate 1329.8 in his opening log entry when the ship is in pursuit of Mudd's vessel. But after Mudd and the women have been brought aboard, Kirk's next log entry says that it is Stardate 1329.1.
Continuity mistake: At the end when the entity left the Enterprise, it came out of the secondary hull, however it left from the engineering dept. The Engineering dept. is located in the back of the primary hull.
Continuity mistake: In the opening scenes, Sulu's retractable viewer isn't there when the Klingon ship approaches on the view screen. But a moment later in a closer shot, the viewer is suddenly in front of him, fully extended. (00:01:05)
Continuity mistake: When Spock orders the phasers to fire, they shoot outward from the ship at an angle from each other, but when they hit the asteroid they converge at a single point. (00:24:00)
Answer: Kirk was getting his physical and Dr. McCoy probably turned off communications, because if he hadn't, Kirk would have left and headed straight for the bridge, leaving McCoy irritated.