Revealing mistake: In the jail cell scene, when Kirk tears a strap from the bed, his left handcuff is open. It remains open for several seconds, while he tries to conceal it by moving his right hand to block the camera's view.
Revealing mistake: When the Enterprise is attacked in the prologue, some of the crew grab onto the central console (that Sulu and Chekov usually sit at). The console noticeably lifts up from the stage surface, almost falling over. Twice.
Revealing mistake: In the middle of the episode Spock goes to Plack Tao to meditate. We see a shot of him and then we see some other people. At this moment look directly behind all of them and you will see Spock waiting for his next shot.
Revealing mistake: Isis the cat was disinclined to hold still for special effects shots. When Gary Seven beams off the Enterprise with the cat in his arms, the position of her head jumps several inches as they're dematerializing. (00:10:10)
The Trouble With Tribbles - S2-E15
Revealing mistake: When Kirk and Spock answer the intercom in the ship's corridor, Spock's uniform has a large brown coffee stain on the front. (This was digitally removed in the DVD release, but is still visible in the newly-syndicated "computer enhanced" version.). (00:17:00)
Revealing mistake: During the fist fight between the transported crew and Spock, the use of stunt doubles in the wide shot is quite obvious.
The Trouble With Tribbles - S2-E15
Revealing mistake: When Kirk finds his position on the bridge occupied by the Tribbles, he calls for Dr. McCoy, and as he arrives, Kirk thrusts some of the Tribbles under his nose. In the shots where Kirk's face is seen, one of the Tribbles he is holding features a visible seam.
Who Mourns for Adonais? - S2-E2
Revealing mistake: When Apollo first grows large, the shimmering edge of the special effects matte is visible in the sky just above his right shoulder. (00:13:35)
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)
Revealing mistake: In McCoy's video of the Capellans, the weapon thrown at the sapling actually misses it and nails a nearby bush instead. The tree, however, obediently breaks in half and falls over anyway. (00:00:20)
Revealing mistake: After being put in the cell, when the camera is in a closeup of Spock, you can see the lines in the prosthetic ears, and can easily tell where the makeup is blended into his real ear.
The Gamesters of Triskelion - S2-E16
Revealing mistake: As the camera does a close up of the Master Thrall, the line of the bald skin cap is visible. (00:05:10)
Revealing mistake: A number of times through the episode, the wire holding Nomad up and moving it around is visible. Example: When Nomad first moves off the transporter pad, look directly above it; the black wire can be seen in front on the light over the pad.
Revealing mistake: Near the end of the episode, after Kirk has destroyed the orb, we see Korob and Sylvia in their true form. As they move, you can see the black strings used to manipulate the puppets.
Revealing mistake: While Nona is struggling with her Villager attackers, the camera tilts just far enough to show, in the distance, a nice (if slightly smoggy) view of buildings, docks and boats in Santa Monica Harbor. (00:45:00)
Revealing mistake: After Scotty finally locates Seven on the gantry while he's tampering with the rocket, it cuts to a quick shot of Kirk and Spock, then in the following shot of Roberta when she turns the vault's combination locks, the stunt wire is visible behind her as it moves up to turn the door's wheel handle. (00:35:25)
Revealing mistake: During the fight between Kirk and Tracey, the outline of the knee pads under Kirk's pants are visible, particularly while Kirk is on the floor with his knees bent.
Revealing mistake: During the televised fight, in the wide angle shots, you can tell it's Spock's stunt double fighting.
A Piece of the Action - S2-E17
Revealing mistake: The drawers of Krako's desk are on the front side, not on Krako's side where they should be.
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.