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.

Continuity: The landing party is out of contact with the Enterprise for two days because of the missing communicators. Wouldn't the Enterprise notice at some point and beam down some more, or make some other attempt to contact Kirk & Co.?

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.

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.

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.

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

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.

Continuity: Kirk and McCoy are in the Exeter's engineering section when Kirk uses the intercom to ask if anyone is aboard. Several shots of the deserted ship follow as his voice echoes off the walls, including one of engineering - with Kirk and McCoy suddenly, mysteriously absent.

Visible crew/equipment: A camera shadow briefly intrudes on the right side of the screen as Kirk asks Mr. Scott about the sensor readings.

Visible crew/equipment: When Kirk turns to follow Miranda across the arboretum, the camera shadow is briefly visible on the wall at the lower left of the screen.

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.

Deliberate "mistake": So that he can look in the right direction, toward Lincoln, a shot of Kirk is reversed before the first battle, putting his uniform insignia on the wrong side.

Continuity: When Plasus and two sentinels beam down to break up the fight near the mine, Plasus is on the end of the trio as they materialize. The camera angle changes as they step off the dais, and Plasus is suddenly in the middle.

Revealing: Because stock special effects footage was used, the shuttle supposedly "stolen from Starbase 4 two weeks ago" is plainly marked "Galileo, NCC 1701/7, USS Enterprise."

Continuity: As Balok's 10 minutes are counting down, McCoy comes onto the bridge wearing a standard blue velour uniform. A few minutes later, when Spock says he can bring up Balok's image, McCoy is suddenly wearing his medical tunic with the different fabric and collar. Then when Bailey starts losing it, McCoy is back in the regular velour costume again.

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: How the heck did the Gideons (who are not members of the Federation) get such exact specifications to make what Spock describes as "an exact duplicate of the Enterprise"? Does Starfleet just hand the plans out to anyone?

Continuity: In one scene the Enterprise's phasers were pink instead of blue.

Continuity: When the android Kirk boards the Enterprise and takes a "command pack," he walks out and down to the transporter, but he doesn't have the pack in his hand.