Revealing mistake: When McCoy is examining Charlie in sickbay, several shots of the diagnostic panel show a clear reflection of Charlie's upright profile. But Charlie isn't sitting up during the exam - he's lying flat on the table. The close-ups of the panel were shot while actor Robert Walker Jr. was either sitting or standing nearby.
Plot hole: Spock claims that history is unchanged at the end of the episode according to the library tapes - how would he know? If history changed, the tapes would change too. And unlike in "City on the Edge of Forever", there's no Guardian around to keep people from being influenced by an altered timeline.
Continuity mistake: When Kirk and McCoy dispute about the tribbles on the Enterprise bridge, Kirk holds the two tribbles in his hands at alternate heights. In the shots where McCoy's face is seen, he holds them next under his chins, but when Kirk's face is seen, he holds them at chest level.
Continuity mistake: When Sarek enters the banquet room to take his medication, the Tellarite sitting at the table holds a glass in his right hand. In all the full shots with Sarek in the foreground, the glass switches to the Tellarite's left hand. In all the close-ups of him, it's back in his right again.
Revealing mistake: After the ship enters the galactic barrier, when Kirk orders "navigation on automatic" there are flashes of light, and in a closeup of the first console just to the left of the main viewscreen, we can see the special effects material spread across the panel below the controls, for the fire to burn when it explodes. Then it cuts to the wideshot as a second console (near Spock's station) blows up, but in the background that first console is intact - no fire/damage whatsoever (it explodes again in a following wideshot).
Continuity mistake: When Uhura tells Kirk that she's made contact with his private transmitter she's seated at the communications console, but after the contact's been broken when she tells Kirk, "It's impossible at the moment because they stopped broadcasting," in this shot facing Uhura the turbo lift is not behind her - she's at a different station on the bridge - then she's back to where she was once again.
Add timeSuper Grover
Visible crew/equipment: On the bridge, Uhura tells Kirk that Outpost 4 reported they're under attack, and after Scott says, "I've talked to my engine room, sir. We'll get more speed out of her," we can see the movement of the overhead boom pole's shadow as Kirk walks toward Uhura.
Add timeSuper Grover
Visible crew/equipment: The action is shifting back and forth from the bridge where the alien ship is attacking and sickbay where McCoy is operating on Sarek. In two separate takes when the scene shifts to sickbay McCoy is standing at the operating table. I noticed a wisp of smoke rising from McCoy's right side (left side of screen). It appears to be from a cigarette in an ash tray below camera level either beside McCoy or near the head of Spock's bed.
Audio problem: During the scene where Kirk is talking in the briefing room, the director was obviously not pleased with how James Doohan said his lines.Due to budget constraints, instead of re shooting the scene, they simply replaced Mr Scotts dialogue with another one.You can see it since at one point his lips are completely out of sync, even on the remastered edition.