During the teaser, Lt. Kyle is wearing a red tunic. After the credits, he's wearing a gold tunic.
When the telemetry probe is launched, the bridge crew cover their ears. Spock, however, does not. Spock is supposed to have more sensitive hearing than humans, so why doesn't he clutch his head in pain?
Spock refers to the organism's size in miles, but Kirk talks about "thousands of kilometers". Not only do they still mix the two types of measurements, but you'd think Spock would be the one that uses "kilometers."
The officer at the helm is the character who has been referred to as Lt. Kyle, and is named that way in the credits. But Kirk keeps calling him "Kowel."
Kirk's mirror in his cabin is still waist high with no chair or anything in front of it.
Kirk: Insufficient data is not sufficient, Mr. Spock. You're the science officer, you're supposed to have sufficient data all the time.
McCoy: I have one. I recommend survival. Let's get out of here.
Kirk: It's a suicide mission.
Spock: You have a martyr complex, Doctor. I submit that it disqualifies you.
McCoy: You think that I intend to pass up the great living laboratory since…
Spock: Vulcans saw it first… and died.
McCoy: Just because the Vulcans failed…
Spock: I am more capable.
Spock: This is not a competition, Doctor. Whether you understand it or not, grant me my own kind of dignity.
McCoy: Vulcan dignity? How can I grant you what I don't understand?
Spock: Then employ one of your own superstitions. Wish me luck.
Spock: Tell Dr. McCoy… he should have wished me luck.
Spock: I have noted the passage of the Enterprise on its way to whatever awaits it. If this record should survive me, I wish it known that I bequeath my highest commendation and testimonial to the captain, officers, and crew of the Enterprise… the finest starship in the fleet.
Kirk: Spock, you're alive!
Spock: Obviously, Captain. And I have some fascinating data on the organism.
McCoy: Don't be so smart, Spock. You botched the acetylcholine test.
Kirk: Later, later, later.
Spock: Captain, I recommend you abandon the attempt. Do not risk the ship further on my behalf.
McCoy: Shut up Spock! We're rescuing you.
Spock: Why thank you, Captain McCoy...
Spock: (Humans) find it easier to understand the death of one than the death of a million. You speak about the objective hardness of the Vulcan heart. Yet how little room there seems to be in yours.
McCoy: Suffer the death of thy neighbor, eh, Spock? Now you wouldn't wish that on us, would you?
Spock: It might have rendered your history a bit less bloody.
Desilu No: 5149-48.