When Spock and Tormolen are being examined at the beginning of the episode, they're both wearing black T-shirts that are clearly supposed to be worn by the male crew members under their long-sleeved shirts, as we clearly see Spock putting his blue shirt over the black one. But later, when Bones rips the shoulder of the Kirk's yellow shirt to give Kirk a hypospray, there's no black shirt underneath, though the uniform has the black collar visible.
When Scotty is cutting through the bulkhead with the phaser he is concerned about cutting through wiring and he doesn't want to go any faster--wouldn't it have been easier and faster to use the phaser to just cut through the door?
Trivia: Tormolen is the Original Series' first, and only, Lieutenant junior grade, as indicated by his single broken uniform braid.
The Psi 2000 virus apparently infects quicker with each new victim. Those infected at the beginning of the episode take a fair amount of time before the virus' full effects are apparent. Most of them exhibit only the annoyance/sweating on the point of contact at first. Later, when Nurse Chapel infects Spock, he immediately feels the full effects. Likewise, when Kirk is infected, he's immediately feeling the full effects.
When Spock enters the briefing room, there is a sign on the door saying, "Briefing room 2". It was on the left side of the door. When they leave the briefing room, the tag is on the right side of the door.
When Scotty was cutting through the bulkhead with the phaser, there was no beam coming out of the phaser. (This was corrected in the 2006 remaster.)
When Dr. McCoy and Nurse Chapel operate on Joe, the medical readings change between the close-up of the screen and the far shot.
When the ship's chronometer runs in reverse at the end, then stops and starts going forward, it reads, "7:59 - 7:00 - 8:01". (This sequence was fixed in the 2006 remaster by replacing the mechanical chronometer with a CGI-rendered version without rotating dials.)
Later, in "Operation: Annihilate" we'll find out that Vulcans have sensitive hearing. Here, Spock totally fails to hear Sulu say he's deserting his post, although Sulu is only about 10' behind him.
The isolation suits Spock and Tormolen wear to the surface aren't...well, isolated. The helmet isn't connected to the suit and there's a noticeable gap.
In the opening shot the planet rotates right to left, but on the viewscreen from the bridge it rotates left to right.
Scotty burns through the bulkhead with a phaser on heat/cut and then immediately grabs the resulting panel and yanks it out with his bare hands - ouch!
Early in the episode, the planet totally disappears from the view screen twice and then reappears.
Spock: Our spectro-readings showed no contamination, no unusual elements present.
Scotty: At least none your tricorders could register.
Spock: Instruments register only those things they're designed to register. Space still contains infinite unknowns.
Joe Tormolen: We're all a bunch of hypocrites. Sticking our noses into something that we've got no business. What are we doing out here, anyway?
Sulu: Take it easy, Joe.
Joe Tormolen: Bring pain and trouble with us. Leave men and women stuck out on freezing planets until they die. What are we doing out here in space? Good? What good? We're polluting it! We're destroying it! We've got no business being out here! No business!
Joe Tormolen: You don't outrank me and you don't have pointed ears so just get off my neck!
Riley: Where's Joe?
Nurse Chapel: Well...
Riley: He died, didn't he?
Nurse Chapel: Yes.
Riley: You know something? You have such lovely eyes, pretty lady.
Nurse Chapel: I know he was a friend of yours. This must be a terrible shock.
Riley: You know what Joe's mistake was? He wasn't born an Irishman.
Riley: Lt. Uhura, you've interrupted my song. Uh, I'm sorry but there'll be no ice cream for you tonight.
McCoy: We're doing everything that's possible!
Kirk: Bones I want the impossible checked out too!
Scotty: We'll be warping out of orbit within a half second of getting your command.
Kirk: I'll hold you to do that half-second, Scotty.
Scotty: Even if we were under full scale attack, I could na move any faster, not in maintaining a safety factor.
Spock: At the rate you're proceeding, you'll take a minute and a half more than we have left. We cannot afford a safety factor.
Kirk: (to Spock) Love. You're better off without it and I'm better off without mine. This vessel. I give. She takes. She won't permit me my life. I've got to live hers.
Nurse Chapel: Mr. Spock... the men from Vulcan treat their women strangely... at least people say that.
Riley: Attention, crew. This is Captain Riley. There will be a formal dance in the bowling alley at 1900 hours tonight.
Riley: (over shipwide intercom) This is Captain Riley, crew. I have some additional orders. In the future, all female crewmembers will wear their hair loosely about their shoulders. And use restraint in putting on your makeup. Women... women should not look made up. And now, crew, I will render "Kathleen" ONE MORE TIME!
Kirk: (muttering) Please, not again.
Sulu: I'll take you, fair maiden!
Uhura: Sorry, neither!
(Spock nerve-pinches Sulu)
Spock: Take D'Artagnan here to Sick Bay.
Riley: (over shipwide intercom) Now, attention, cooks. This is your captain speaking. I would like double portions of ice cream for the entire crew.
Scotty: Ah canna' change the laws of physics. I've got to have thirty minutes.
Scotty's line "I canna change the laws of physics!" (when realizing that Riley had shut off the engines) would be later be used as his character's line in the 1987 parody song Star Trekkin.
John D.F. Black originally planned for Sulu to act out a samurai fantasy. George Takei felt it would be stereotypical and suggested the Musketeers fantasy would be more interesting.
This episode marks the first appearance of Nurse Christine Chapel (Majel Barrett). She would go on to become a recurring character through all three seasons of the show and appear in two movies. Here she receives top billing in the end credits, over even DeForest Kelley and James Doohan.
Lt. Riley declares that "a formal dance will be held in the bowling alley at 1900 hours tonight." However, he is also delusional, so it's not certain that the bowling alley he spoke of actually existed. The bowling alley was included in the 1975 Franz Joseph deck plans of the Enterprise (published by Ballantine Books), probably based on this mention.
This episode was originally supposed to be the first part of a two-part episode that ended with "Tomorrow is Yesterday", but both episodes were rewritten when the idea was dropped.
The decontamination suits that Spock and Tormolen wear to the planet are made of shower curtains.
Spock: Take D'Artagnan down to sickbay.
Sulu seems to think that he is the main character in a series of novels written by Alexandre Dumas that follows the adventures of a young man who leaves home to join The Three Musketeers. He sees Kirk as Cardinal Richelieu, the powerful and cunning advisor to King Louis XIII depicted as a power-hungry and greedy minister in the novels.