This episode was the second pilot filmed for the network to try and sell the series.

Leonard Nimoy (Spock) is the only actor to appear in both this episode and the first pilot, "The Cage".

The Enterprise sets used in this episode were originally built for "The Cage".

This is the only appearance of the phaser rifle on the series.

This is the only episode in which Leonard Nimoy (Spock) wears a gold uniform instead of his standard blue one.

This is the only episode of the series in which James Doohan] (Scotty) appears but DeForest Kelley (Dr. McCoy) does not.

Mitchell mentions that he once steered a "little blond lab technician" over to Kirk whom he nearly later married. Many fans believe this to be Dr. Carol Marcus, one of Kirk's many ex-lovers and a major character in Star Trek: The Wrath of Khan.

Roddenberry intended to use "Rice" as Kirk's middle name, explaining why the initial R appears on the tombstone. Roddenberry included a character named Rice in his previous shows.

Kirk: Mr. Spock, has anyone ever told you that you play a very irritating game of chess?
Spock: "Irritating"? Ah, yes, one of your Earth emotions.
(Kirk makes a threatening move)
Kirk: Certain you don't know what irritation is?
Spock: The fact that one of my ancestors married a human female...
Kirk: Terrible having bad blood like that.

Spock: (Captain Kirk) played most illogically - his next move should have been the rook.

Dehner: lf there was an emergency, I'd be interested in how that crew reacted, too.
Mitchell: Improving the breed, Doctor?
Dehner: Is that your line? I heard that's more your specialty, Commander, line included. (walks away)
Mitchell: Walking freezer unit.

Mitchell: Hey, man, I remember you back at the Academy; a stack of books with legs! The only thing I ever heard from an upperclassman was, "Watch out for Lt. Kirk! In his class, you either think, or sink!"
Kirk: I wasn't that bad, was I?
Mitchell: If I hadn't aimed that little blond technician at you...
Kirk: You what?! You...you planned that?
Mitchell: Well, you wanted me to think, didn't you? I outlined her whole campaign for her!
Kirk: I almost married her!
Mitchell: Better be good to me. I'm getting even better ideas here.

Dehner: Women professionals do tend to over-compensate.

Dehner: A mutated, superior man could also be a wonderful thing... the forerunner of a new and better kind of human being!

Kirk: Dr. Dehner feels he isn't that dangerous – what makes you right and a trained psychiatrist wrong?
Spock: Because she feels--I don't. All I know is logic. In my opinion, we'll be lucky to repair the ship and get away in time.

Mitchell: You should have killed me while you could, James... command and compassion are a fool's mixture.

Mitchell: Time to pray, Captain. Pray to me.
Kirk: To you? Not to both of you?
Mitchell: Pray that you die easily.
Kirk: There'll only be one of you in the end. One jealous god... if all this makes a god. Or is it making you something else?
Mitchell: Your last chance, Kirk.
Kirk: Do you like what you see? Absolute power corrupting absolutely.

Kirk: I want (Mitchell's) service record to end that way. He didn't ask for what happened to him.
Spock: I felt for him, too.
Kirk: I believe there's some hope for you after all, Mr. Spock.

On the headstone created for him by Mitchell, the name reads "James R. Kirk." In numerous later episodes, his middle initial was revealed to be "T".

This episode boasts the highest number of Enterprise fatalities at 12: the nine dead crewmen at the beginning, Kelso, Dehner, and Mitchell.

Although never made official, it is commonly assumed that the "blonde lab technician" referenced here is Dr. Carol Marcus, who Kirk later ends up having a son with. The non-canon novels make reference to this "fact."

What Changed in the Remastered Version
Opening and closing shots feature the Enterprise (CGI model) transiting a nicely crafted nebular cloud. The Galactic Barrier gets a very impressive CGI enhancement. So do the orbital view and surface shots on Delta Vega, with cloud effects and better matte shots contributing to a more realistic planetary surface feel. The establishing shot of the lithium cracking station gets some animation showing Kirk and Dehner entering the facility, linking the scene to the preceding scene more naturally. The phaser rifle shot at Mitchell gets an upgrade featuring a more solid beam and a multicolored splash.