This is the first of many Star Trek episodes to feature a transporter malfunction.

This is the first episode to feature the Vulcan nerve pinch. It is also the first time that Dr. McCoy says "He's dead, Jim".

This is the first of several times that Captain Kirk is presented with a duplicate of himself.

This is the first episode to feature the captain's more relaxed wraparound tunic. Here it is used to differentiate the two Kirks.

Spock: Is there something I can do for you, Captain?
Kirk: Like what?
Spock: Well, Dr. McCoy seemed to think I should check on you.
Kirk: That's nice. Come on, Spock, I know that look. What is it?
Spock: Well, our good doctor said that you were acting Iike a wild man, demanded brandy.
Kirk: Our good doctor's been putting you on again.
Spock: Hmm. Well, in that case, if you'll excuse the intrusion, I'll get back to work.
Kirk: I'll tell him you were properly annoyed.

Kirk: You're too beautiful to ignore. Too much woman.

Kirk: I'll make an announcement to the entire crew, tell them what happened. It's a good crew. They deserve to know.
Spock: Captain, no disrespect intended, but you must surely realize you can't announce the full truth to the crew. You haven't the right to be vulnerable in the eyes of the crew. If you're anything less than perfect, they lose faith, and you lose command.
Kirk: Yes, I do know that, Mr. Spock. What I don't know... is why I forgot that just now.

Kirk: How's it going down there, Mr. Sulu?
Sulu: It's already 20 degrees below zero. Can't exactly call it balmy.

McCoy: You have a point, Spock?
Spock: Yes. Always, Doctor. You have here an unusual opportunity to appraise the human mind, or to examine, in Earth terms, the roles of good and evil in a man. His negative side, which you call hostility, lust, violence; and his positive side, which Earth people express as compassion, love, tenderness. And what is it that makes one man an exceptional leader? We see here indications that it is his negative side which makes him strong--that his evil side, if you will, properly controlled and disciplined, is vital to his strength. Your negative side, removed from you, the power of command begins to elude you.

Spock: If I seem insensitive to what you're going through, Captain, understand--it's the way I am.

Sulu: Can you give us a status report, Captain? Temperature's still dropping. Now 41 degrees below zero.
Kirk: We've located the trouble. It shouldn't be much longer.
Sulu: Could you find a long rope somewhere and lower us down a pot of hot coffee?
Kirk: I'll see what we can do.
Sulu: Rice wine will do, if you're short on coffee.

McCoy: We all have our darker side. We need it; it's half of what we are. It's not really ugly, it's human.
Kirk: Human.
McCoy: Yes, human. A lot of what he is makes you the man you are. God forbid I should agree with Spock, but he was right. Without the negative side, you couldn't be the captain. Your strength of command lies mostly in him.

Spock: Being split into two halves is no theory with me, doctor. I have a human half, you see, as well as an alien half... submerged, constantly at war with each other. I survive it because my intelligence wins out over both, makes them live together.

McCoy: How do you feel, Jim?
Kirk: How? I've seen a part of myself no man should ever see.

Why don't they use the shuttlecraft to bring up the stranded landing party? This early in the series it's still been established that the hangar bay is at the back of the secondary hull, and they have six shuttlecraft.

On the bridge when the savage Kirk arrives, Farrell's insignia disappears and then reappears.

On the bridge, the savage Kirk's scratches jump to the opposite (right) side of his face in two closeups.

The bestial Kirk takes a Type II pistol phaser from Walker, and he's holding it in the close-ups in Engineering. However, in the far shots he's holding the Type I phaser.

Kirk makes a log entry citing Stardate 1673.5. However, later after the animal dies Spock makes a log citing Stardate 1673.1.

In his log entry, Spock refers to himself as second officer.

In the first few minutes of the episode, Kirk and Sulu are missing their Enterprise chest insignia. Originally the filming was "flipped" to make the set look bigger from different angles. The flipping was removed from the DVD and re-mastered releases, but since they had originally removed the insignia so they wouldn't be reversed when the film was flipped, the insignia couldn't be put back in. in the original version with the camera flipping, Kirk's hair part can be seen to reverse as well.

Heating units malfunction when beamed down, due to the duplication process. However, they don't beam down non-powered heating gear, such as... warm coats and blankets. Instead Sulu and the others shiver to death under thin tarps.

Even this early in the series when Spock is still evolving as a character, his tactless comment to Rand about the counterpart is wildly out of character, as a Vulcan, as an officer speaking to a crewman, and as any civilized being talk to a woman who was almost raped.

Oddly, they don't beam up the landing party before the transporter is fixed. Given the chance between sure death by freezing and an eventual death (with time to find a cure) due to being split apart, the latter seems indisputably preferable.

Scotty calls Kirk and Spock to tell them he's discovered the cause of the transporter breakdown. They were there and indirectly caused it: why didn't they tell him?

Despite the fact Scotty says it will take at least a week to fix the transporter, he and Spock have it fixed only a few minutes after the commercial break. One wonders why the writers even put in this dramatic contrivance: it doesn't further the plot, and is simply brought up and immediately dispatched.

What Changed in the Remastered Version
General improvements cited on the main series page. The orbital shots of the planet get a a surface upgrade and make it much clearer it's an ice planet. Phaser beams are also upgraded to the standard remastered animation. The split screens of the two Kirks are cleaned up a bit, although there's still a noticeable "jump" when positive Kirk and Spock enter the elevator, it closes, and negative Kirk extends his hand into the camera shot.