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.
Thelin: This change in the timeline will put you in my place, yet I'm not aggrieved.
Spock: Andorians are not known for their charity.
Thelin: True. A warrior race has few sympathies, but one we do possess is for family. In your time plane, you will live and so will your mother. That is valuable. Live long and prosper in your world, Commander Spock.
Spock: And you in yours, Commander Thelin.
Sarek: My apologies, visitor. I regret you were witness to that unfortunate display of emotion on the part of my son.
Spock: In the family, all is silence. No more will be said of it. Live long and prosper, Sarek of Vulcan.
Sarek: Spock, being Vulcan means following disciplines and philosophies that are difficult and demanding of both mind and body.
Young Spock: Yes, Father.
Sarek: You constantly display your emotions. You have even been seen fighting in the street.
Young Spock: Yes, Father.
Sarek: The time draws near when you will have to decide whether you will follow Vulcan or human philosophy. Vulcan offers much. No war, no crime. Order, logic, and control in place of raw emotions and instinct. Once on the path you choose, you cannot turn back.
Young Spock: Yes, Father.
Young Spock: Thank you for helping me and I-Chaya.
Spock: It was my duty, Spock.
Young Spock: Mother says you should always say thank you.
Spock: The lady Amanda is known for her graciousness.
Young Spock: Do you think I'll ever be able to do that neck pinch as well as you?
Spock: I dare say you will.
Spock: There is some human blood in my family line. It is not fatal.
Healer: You are Spock, son of Sarek, are you not?
Young Spock: Yes, Healer.
Healer: I have heard of you. I have heard of a tendency toward what humans call "practical jokes."
Young Spock: I did that once, two years ago. Healer, I would not call you out unless a life was in danger. Have you ever heard the son of Sarek was a liar?
Young Spock: I wanted only to help I-Chaya. He was my father's before he was mine. To lose him...
Spock: A Vulcan would face such a loss without tears.
Young Spock: How?
Spock: By understanding every life comes to an end when time demands it. Loss of life is to be mourned but only if the life was wasted. I-Chaya's was not.
Young Spock: If you will excuse me now, I have some business to conduct with schoolmates.
Young Spock: A demonstration of the Vulcan neck pinch.
Kirk: I sent the others up to the ship. What happened?
Spock: One small thing was changed this time. A pet died.
Kirk: A pet? Well, that wouldn't mean much in the course of time.
Spock: It might to some.
McCoy: Never mind the chitchat. I've got my medical scanners all set up for a Vulcan. I have to recalibrate every time I run a physical on you, Spock.
Spock: Dr. McCoy, you do not know your good fortune. If the times were different, you would have to recalibrate for an Andorian.
McCoy: What's that supposed to mean? If that was supposed to be a joke, Spock, I have to remind you Vulcans don't tell jokes.
Spock: Times change, Doctor. Times change.
The Le-Matya's roar is the same sound effect as the roar of Toho Films' kaiju, Godzilla.
George Takei and Nichelle Nichols are credited but do not lend their voice talents to the episode.
A wild version of a sehlat similar to that kept by the young Spock as a pet, was seen by Captain Archer and Commander T'Pol while traveling on Vulcan in the Star Trek: Enterprise episode "The Forge."
The sentient time portal the Guardian of Forever first appeared in the Star Trek (Original Series) first-season episode "The City on the Edge of Forever". Originally voiced by Bart LaRue, he is played by James Doohan in this episode.
Spock's pet sehlat was first mentioned by his mother Amanda in the Star Trek (Original Series) second season episode "Journey to Babel."
This is the first time in Original Trek or anywhere else we hear of Amanda's maiden name: Grayson.
Keith Sutherland (Sopek) is the son of director Hal Sutherland, who directed this and many other animated episodes.
Billy Simpson never actually recorded for this episode, or worked with Leonard Nimoy or anyone else. All of the material of Young Spock comes from Simpson's audition tape, which was edited in later and combined with Mark Lenard, Leonard Nimoy, etc.
When story editor D.C. Fontana was writing this episode she ran into opposition from the network executives when they learned her story would have Spock's beloved pet mortally wounded and requiring euthanasia. The executives were concerned that it would upset young viewers and insisted that the pet be allowed to live. Gene Roddenberry supported Fontana and reassured the management that she would handle the story appropriately. As it turns out, there were never any complaints about the episode and the story is considered the finest of the series.
Author D.C. Fontana penned the original episodes "Tomorrow is Yesterday", "Charlie X", "This Side of Paradise", "Journey to Babel", "Friday's Child", "By Any Other Name", "The Ultimate Computer", and "The Enterprise Incident". Purportedly she did much behind-the-scenes script cleanup work as well and is generally accredited as one of the "founding fathers" and major creative forces behind Star Trek.
The actors who played Sofek and Stark are unknown.
Since Jane Wyatt wasn't available on the show's budget, Majel Barrett fills in for Amanda.
Mark Lenard reprises his role as Sarek from the original series.