Revealing mistake: In Trelane's castle, Kirk and Sulu are supposed to be frozen, but they slightly waver.
Revealing mistake: Just before Kirk finds himself in Trelane's "courtroom", look behind Kirk on the bridge above Spock's work station. The viewscreen with the planet on it is actually a piece of paper that's coming away from the wall.
Revealing mistake: At different points during the Metrons' transmission to the Enterprise, both Kirk and the Gorn pop into the picture a few seconds after the rocky landscape appears. (00:37:00 - 00:37:55)
Revealing mistake: As Kirk and the Gorn battle, they briefly confront each other from opposite sides of a dead tree which has truncated limbs. One of the larger tree limbs on this supposedly uninhabited, tiny world has obviously been cut off with a chain saw or similar cutting instrument.
Tomorrow is Yesterday - S1-E20
Revealing mistake: Traveling towards the Sun and away from the Earth, parts of the Enterprise are missing due to superimposing the model into the current shot. As the Enterprise goes by, one of the warp engines seems to be disintegrating at the rear. (00:42:05)
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.
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.
The Return of the Archons - S1-E22
Revealing mistake: The rocks pelting Kirk and the landing party when all hell breaks loose bounce when hitting the actors, the brick walls, and the concrete sidewalk.
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.
Space Seed - S1-E23
Revealing mistake: During the fight sequence between Kirk and Khan in the engine room, you can see in some shots that it is not William Shatner and Ricardo Montalban fighting, but their stunt doubles.
Space Seed - S1-E23
Revealing mistake: Just before Kirk and Khan fight in Engineering, Khan bends Kirk's phaser pistol in half, but the small hand phaser on top is still functional.
A Taste of Armageddon - S1-E24
Revealing mistake: It is easy to tell where the matte painting and live action area meet when Fox and the assistant appear.
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)
Revealing mistake: When Kirk holds a golden egg, a seam can be seen on it. (00:43:55)
Revealing mistake: A straight and level seam connecting the top and bottom halves of the tunnel Spock is crawling through is visible.
Suggested correction: Views of the tunnels made before the creature was wounded by Kirk and Spock appear almost perfectly smooth. It is explained that the creature exudes a powerful acid to dissolve the rock. This tunnel was made after the creature was wounded, so it is logical that the wounded portion of the creature would secrete less acid thus leaving an imperfection as the creature tunnels. This could be a case of incredible attention to detail by the set designer rather than an error revealed.
This correction is too much of a stretch to explain a perfect seam by the wounded Horta. Plus, if the Horta was secreting less corrosive substance, then that area would be less eroded, not more. If attention to detail was paid, then the area would have an outward seam, not an inward one.
Operation -- Annihilate! - S1-E30
Revealing mistake: When Kirk and company beam down, the trees are brown. In the next shot, the trees are green. Most probably a different camera lens was used for the shots. (00:07:10)
Operation -- Annihilate! - S1-E30
Revealing mistake: While Spock and Kirk discuss the phaser resistant creatures, Kirk declares that it may be a trap and to move out. When one of the flying parasitic creatures attacks Spock, in his first closeup just as Spock turns around we can see the thin wire leading from the creature to the right side of the screen, and the wire also casts a shadow on Spock's uniform. (00:18:10)
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.
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)
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.
