Deliberate "mistake": Special effects of the ship in space were very expensive in the 60s and couldn't be wasted. So when a larger model was built with slightly different nacelles, shots of both versions became common, even within the same episode. This is why the Enterprise sometimes had red needle-tipped nacelles and sometimes lighted "spinning" ones, and in aft views she had either round white balls or perforated vents at the nacelles' ends.

Visible crew/equipment: "The Cage": When the doctor is mixing the drink and Pike gets up off the bed to accept it, an equipment shadow sweeps across the left side of the screen.

Plot hole: "The Cage": Pike says he doesn't want women on his bridge, Number One being the one and only exception, and he keeps ordering Colt off it. His apology to Number One indicates that A)he doesn't think of her as female, and B)she's the only exception to his rule. So how does he overlook the very female crew-woman seated at the science station?

Revealing: "The Cage": When the laser cannon starts firing at the Talosian rockface, a styrofoam block falls and rolls away on the left side of the screen. After a brief shot of the crew, we see the cannon still firing, and the exact same block falls and rolls off screen on the left again.

Revealing: While the interior sets for the shuttle craft provided adequate room for the actors to stand, the mock-up built for exterior shots was undersized, and too short to stand up in by several inches. So when entering or exiting the shuttle, the actors always had to duck.

Revealing: "The Cage": In the first illusion, Vina runs down the path from the fortress to Pike. When we cut to close-ups, they're both suddenly standing much closer to the building than before, and the fortress has changed color from brownish orange to white.

Continuity: "The Cage": When Pike threatens the Keeper and is stricken with pain, both Yeoman Colt and Number One grab his arms. In the next shot, however, he's standing clear of them with one hand on the glass. Next shot after the Talosian leaves, they're both holding his arms again.

Revealing: "The Cage": Due to a poor edit, there's an odd jump cut and a change of lighting just as the Keeper comes out of the elevator on his way to an attempt to steal the laser pistols from the cell.

Continuity: When Rand brings Sulu the plate of food, it has a bowl of colored cubes on it. Then there's a cut to a close-up of Sulu eating the cubes and the bowl is now a plate. Then in the long shot it's a bowl again.

Continuity: When they first arrive on the planet, the tricorder Kirk is holding for McCoy starts on his left shoulder, disappears in the next shot, then in the next shot after that reappears in his right hand.

Other: Spock and McCoy have a brief argument over whether Charlie could have survived on his own all those years. Because they're talking at the same time, Spock saying "Doctor" as McCoy says "Mr. Spock," the closed captioner apparently misunderstood McCoy's line. The caption has him calling Spock "Dr. Spock."

Continuity: When Capt. Kirk and Charlie enter the turbolift heading for the bridge, Kirk is wearing one style of tunic and when the turbolift arrives on the bridge, Kirk is wearing a completely different tunic.

Continuity: Kirk first says the Antares is a cargo vessel, but after it explodes he calls it a science probe vessel.

Visible crew/equipment: In the scene where Dr. Dehnar is talking to Gary Mitchell in sick bay, when he grabs her arm you can see the shadow of the boom mike on the door and adjacent wall.

Continuity: When Kirk is fighting Gary Mitchell, his tombstone reads James R. Kirk. Kirk's middle name is Tiberius. Mitchell is a friend of long standing and would not have got it wrong.

Continuity: Lt. Joey said to Lt. Sulu "You don't outrank me and you don't have pointed ears" but Joey is a junior grade lieutenant so Sulu would outrank him.

Continuity: As the bridge crew monitors Psi 2000 on the view screen, a glitch in the special effects matte makes the planet vanish briefly and then reappear - twice.

Continuity: Initially, the Enterprise orbits Psi 2000 as the planet turns westward. But in later shots, on the bridge view screen, the planet has reversed directions and now turns eastward.

Revealing: When Kirk beams up from the planet, the insignia on his shirt is not there.

Continuity: Near the end, when the evil Kirk is confronting the good Kirk on the bridge and we see him in a close-up, the scratches on his face are on his right cheek. This is wrong because Yeoman Janice Rand had scratched him on the left cheek.