Continuity mistake: Kirk calls Scott to beam up, and lowers the communicator from his face as he looks at McCoy. In the next shot, he's holding it to his mouth once more. The shot changes one more time, and it's lowered again. (00:31:55)Jean G
Continuity mistake: The giant rock sitting on the promontory, in long shot, is much larger than the Kirk stand-in stunt man who climbs the rock face to reach it. Next shot, when Kirk pushes the boulder over the edge, it's suddenly much smaller than he is. When it lands on the Gorn, the rock is even smaller - now less than half its original size. (00:31:35)Jean G
Revealing mistake: Zarabeth tells Spock that Zor Khan only left her what was necessary to survive in the ice age. Mascara, eyeliner, lipstick and hairspray were apparently considered survival necessities. Yes, we know the actress "had" to wear make-up. But in a prehistoric setting, it shouldn't look as though she did. She could and should have been given a more "natural" make-up job - and an unlacquered hairdo. (00:38:25)Jean G
Continuity mistake: Towards the end of the episode when the Enterprise is moving leaving from the starbase, the ship is at an angle to the camera pointing to the right of the screen. The starbase behind it gets smaller and goes up a little. If the Enterprise were leaving the starbase at that angle, the starbase should have moved to the left and then completely off the screen.
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)Jean G
Other mistake: At the end, Kirk teases Spock twice specifically about letting himself be hit on the head in order to determine which Kirk was genuine. But the phony Kirk never hit Spock on the head. He merely pushed Spock over and attacked the real Kirk. Clearly, the action sequence wound up differing from the scripted dialogue, and no one noticed. (Also clearly, Kirk is not speaking metaphorically here.) (00:45:30 - 00:49:20)Jean G
Continuity mistake: While Kirk and McCoy attack the other two henchmen, Spock shoulder-pinches the card-playing gangster. In the very next shot, Spock's position has jumped from behind the now-unconscious man to several feet away, where he is bending over to retrieve the guns from the floor. (00:16:30)Jean G
Continuity mistake: Mr. Leslie appears as one of the guards in the space station, Stardate 45XX.X. Unfortunately, he was killed in 'Obsession,' Stardate 36XX.X. External websites have a variety of excuses for this miraculous reappearance, but he very definitely dies in the show, with no canonical reason for him reappearing beyond it being a mistake.
Continuity mistake: In a few episode we'll hear about "Hodgkin's Law of Parallel Development," but here Spock claims the odds of a planet developing similar to Earth's Nazi culture are "virtually impossible." Why was it so likely as to be easily accepted there, but impossible here?