Revealing mistake: A re-used sickbay scene taken from an earlier episode causes Nurse Chapel to instantly revert to her old hairdo as the sound waves make her fall unconscious. (00:41:00)
Revealing mistake: On the surface of Eden, Spock finds Adam dead under a fruit tree. But the corpse's fingers are still moving. (00:45:20)
Revealing mistake: As Scotty is cutting through the wall, the line where the burn is supposed to go is outlined on the inside of the wall.
Revealing mistake: As the camera is close to Dr. Sevrin, the line of the bald skin wig is easily seen.
Revealing mistake: When Kirk pushes Plasus against a rock wall in the mine, a huge upper section of the "solid rock" wobbles. (00:43:20)
Revealing mistake: When he kicks Colonel Green away during the first fight, Kirk splits his pants. Fortunately, they mend themselves a few shots later. (00:28:00)
Revealing mistake: When Kirk & Spock beam down with Lincoln to the supposedly newly-created non-volcanic landscape, there are footprints all over the ground. They disappear a few shots later, though.
Revealing mistake: When Abraham Lincoln shoves the Klingon away from him during their fight, the Klingon collides with a heavy boulder which moves several inches at impact.
Revealing mistake: About 4/5 through the show, while the Vulcan is being killed, if you look at Lincoln's wrist, you can see his makeup line.
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)
Revealing mistake: For the final time in "Star Trek's" 3-season run, Chekov mysteriously vanishes from the bridge to be replaced by that tall blond doppelganger, thanks to an old piece of 1st season stock footage used for the special effects insertion of the view screen. The costumes are also noticeably different from 3rd season's uniforms. (00:14:05)
Revealing mistake: During the Court Martial scene, a directorial error led to Kirk's exit from the briefing room in the wrong direction. He walks off screen into what would have been a solid wall or corner. The briefing room's only door is on the other side of the room. (00:39:15)
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.