Revealing mistake: 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. (00:12:30)
Visible crew/equipment: When Spock walks into the briefing room, he stands in front of the closed doors telling himself that he's in control of his emotions, and when he says, "My duty," the shadow of the boom mic can be seen moving across the wall at the top of the screen. (00:36:40)
Character mistake: 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.
Visible crew/equipment: As Spock swings the scanner around, as it points toward the camera position, the camera equipment is reflected on the chrome plate on the front.
Other mistake: In the Briefing Room, while Kirk is feeling the effects of the disease, Spock tells Scotty that he will call the restart formula in from the bridge. But just a moment later, Kirk enters the Bridge, and Spock is in the Engine Room working on the settings and formula with Scotty.
Continuity mistake: In the opening, the crewman gets a few drops of the infectant on his hand, but the reddish mass on the wall remains. On the ship, when the spectral view is shown, the red mass is nowhere to be seen.
Continuity mistake: As Scotty starts prying the panel open, it's with a tool in his right hand, and the thumb of his left. After the cut, he's suddenly holding it in his right hand.
Other mistake: After Scotty makes his remarks to Spock, he raises his phaser. The sparks start up before Scotty levels his phaser.
Continuity mistake: When Sulu first emerges on the Bridge, Kirk confronts him, and standing a couple feet from the rapier. A second later, Kirk's a few inches from it.
Revealing mistake: As Spock gets to to the helm position, the whole console visibly wobbles as he hits the controls.
Continuity mistake: As Christine goes to put the respirator in place, she uses her right hand. After the cut, it's her left that puts it in place.
Continuity mistake: As Bones works on Joe's injury, a red light turns on over his head shining down on the area, illuminating his hair. A second later, the red is gone, then back.
Continuity mistake: The indicators radically change positions when Spock gets on the bio bed.
Other mistake: In the Transporter room, as the Decontamination field is used, the shuttered lighting casts not only on Spock and Joe, but on the walls surrounding the chamber.
Continuity mistake: When Sulu confronts the men in the corridor, his epee goes from perfectly straight to having a slight crook, then back to straight when he appears on the bridge.
Continuity mistake: As Joe draws back and stands up, he pulls a knife. First, it's sharp side up, then it's suddenly blade down.
Continuity mistake: When "good" Kirk beams up from the planet, the insignia on his shirt is not there. Then it appears when he's walking with Scotty. (00:03:00 - 00:05:25)
Continuity mistake: When the "evil" Kirk appears, the Enterprise emblem is missing from his shirt. Then later, when he gets the brandy, the insignia is there. (00:04:45 - 00:06:20)
Suggested correction: When Kirk and Sulu are on the planet, they're shown not wearing the insignia. When "good" Kirk beams up, he too doesn't have the insignia. We just don't know where it came from, it could have been in their pocket.
The insignia is not removable, it's permanently attached. They wouldn't be able to remove it. The idea of a removable insignia wasn't introduced until STTNG.
While it might have been sewn on for production purposes, I saw this episode as indicating it was removable given the fact that they weren't wearing one on the planet. Plus, there have been other officers whose insignia is not sewn on in TOS.
Other mistake: Spock records a log entry as the "second officer". It appears they got "first officer" confused with "second in command." (00:36:15)
Continuity mistake: All during the episode there are scratches on the left cheek of the "evil" Kirk. When the "Good" Kirk and the evil Kirk confront each other on the bridge, there's a shot where the scratches are on the right cheek of the evil Kirk.
Suggested correction: This wasn't a continuous shot though, we just don't know where they get the insignia, it could have been in his pocket. Both Kirk and Sulu are on the planet's surface without their insignia. And when "evil" Kirk beams up, he too doesn't have it on, even though later he does.
Bishop73
What possible reason could there be for the Enterprise Insignia, which is stitched to every uniform, to be in 'Evil' Kirk's pocket? No character has done this in any episode of Star Trek. They don't get them from anywhere, they are a part of their uniforms.