When the alien ship's bridge starts to explode, Kirk and the others are lying on the ground. When they materialize back on board the Enterprise they are standing up.
When the landing party beams over to the alien ship, in a number of shots the yellow force field "outline" is badly off-center, with gaps on the left or right and an overlap on the character body on the opposite side.
Kirk tells Sulu to use the "mutual override" to regain control of the ship. The term he should use is "manual override."
Arex is shown seated at the navigator position next to Sulu for the most part, but in several shots a brunette red-shirt is seated in the position instead, before the camera cuts back and Arex is there again.
When Kirk orders the forward scanners on to see the dead star, the viewscreen "whooshes" and slides open vertically, as if there is a shutter over it. It is not a shuttered window.
In one shot in the transporter room, Scotty's insignia can just be seen to be on the wrong side of his uniform.
In one shot in the briefing room, Spock's insignia appears on the wrong side of his uniform.
In one shot of the Andorian Thelin in the Enterprise's briefing room his collar is blue instead of black.
Time-travelling Spock establishes himself with the cover story of being a distant relative (which everyone accepts so must be true), but later tells his younger self that he also has human blood. In the show Spock has always been presented as the first, unique half-Vulcan/half-Human. Wouldn't young Spock think it odd that there were another half-Human Vulcan when he's the first?
The Le-Matya's head briefly disappears when Spock applies a nerve pinch.
Thelin's skin is greyish-pink: Andorians in the series have blue skin.
Lt. Gabler is seen here for what will be the first of four apperances. However, due to color errors, he appears as Caucasian although all of his subsequent appearances establishes confirm the character is African-American.
Trivia: Kirk's reference to having said "I will not kill today" is from the original series episode "A Taste of Armageddon."
When they first receive the signal, Kirk orders it broadcast over the ship's systems, as is clear because there are shots of everyone else listening. But... why? There's no reason everyone on the ship needs to hear it and there's no indication the ship somehow forces people to rebroadcast it.
Held at phaser point by Uhura, Dara asks the computer for the location of the hiding Enterprise officers...and gets the answer. So...why didn't Dara just ask the computer earlier while she and her followers were doing a search for them and couldn't find them?
As Nurse Chapel opens the door to the room where Spock is, the left arm of her uniform is red instead of blue.
Nurse Chapel is wearing a red uniform when she beams down, but a blue uniform in all prior and subsequent shots.
This planet has the same name (Taurus II) as the barren planet Taurus II in the original Trek episode "The Galileo Seven." Since this planet is being explored for the first time in this episode, it can't be the same planet.
The tribbles in this episode are pink. (The goof resulted from the fact that director Hal Sutherland is color blind and was unaware of the color change).
Late in the episode Koloth explains that they need to recover the glommer to avoid an ecological disaster. But this doesn't match with his earlier actions, where he deliberately and repeatedly tries to destroy Jones' scout ship. If it wasn't for Kirk, Koloth would have destroyed the very thing he was told to retrieve.