Revealing mistake: As Charlie watches the pole lowering into a square hole in the deck, the wood grain of the set piece is visible. The wood has been painted gray/silver to look like metal. (00:06:06)
The Gamesters of Triskelion - S2-E16
Revealing mistake: When Kirk goes to kick his opponent, you can see the kick was at least a foot and a half from the man.
Revealing mistake: As Spock gets to to the helm position, the whole console visibly wobbles as he hits the controls.
Revealing mistake: When Mr Hengist goes on the rampage in the hearing room, you can see that the actor playing Hengist is an obvious stunt double wearing a bald cap, whereas John Fiedler has hair on the back and sides.
Revealing mistake: As the camera is close to Dr. Sevrin, the line of the bald skin wig is easily seen.
Revealing mistake: Kirk uses a sword to cleave a Roman guard during his escape from the TV studio, but there is no blood on it. Later, though, when Claudius stabs Merrick in the back, there is blood on the knife blade.
Revealing mistake: Kirk uses a submachine-gun to shoot the lock of the cell holding Spock and McCoy, but no bullet holes appear, nor is the lock damaged.
Revealing mistake: A Roman guard empties half a magazine of bullets from his submachine-gun into Flavius, but there are no bullet holes in his body, nor blood on the floor from the wounds inflicted by the bullets.
Let That Be Your Last Battlefield - S3-E15
Revealing mistake: As Kirk turns all the way around to look at the newly arrived Bele, the shadows of the stage wiring is easily spotted at the top of the screen.
The Return of the Archons - S1-E22
Revealing mistake: In the holding cell, as Spock attempts to analyze McCoy's mental state, the alcove is brightly lit a hand held lamp. As the camera pulls back, the light is seen to move, and then is turned off.
Revealing mistake: As Kirk turns to run on the castle steps, an iron bar barricade appears, blocking his path. When it does, watch the castle wall, and the barricade. It seems to slide just a few inches to line up correctly in the shot, showing poor editing.
The Corbomite Maneuver - S1-E11
Revealing mistake: In Kirk's quarters, as he touches the desk top comm panel, it noticeably slides a little rather than being fixed in place.
Where No Man Has Gone Before - S1-E4
Revealing mistake: The readers in Sick Bay are supposed to be display screens, a type of CRT, for reading material. Mitchell turns his off to rest, but the words are still plainly visible.
Who Mourns for Adonais? - S2-E2
Revealing mistake: As Apollo's head and neck appear on the screen, the neck of the black T-shirt used to mask his body is easily seen, and his shoulders are faintly visible.
A Piece of the Action - S2-E17
Revealing mistake: As Kirk and Spock take off in the car, the painted backdrop is recognizable between the buildings. There is a vertical edge to it about three feet away from the edge of the auto repair building.
Revealing mistake: In the close up shots of the mirror Sulu, the poor make up job of the scar on his face is very evident.
Revealing mistake: As Kirk fights Finney in the Engine room, in the long shots, Kirk's stunt double is easy to pick out from the close shots.
Tomorrow is Yesterday - S1-E20
Revealing mistake: As the camera looks at the pilot in the cockpit, there are supports on either side of and behind him as well as no clouds where there should be, showing he is on a sound stage set.
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.