When Irina comes to visit Chekov in Auxiliary Control, she tries to find out more about it. However, she acts as if she doesn't have a clue what it does, despite the fact that she attended Starfleet Academy for at least some period of time.
Sulu tells the captain that they're leaving the Neutral Zone and headed into Romulan space. But when did they get into the Neutral Zone? It's a violation to even be in the Neutral Zone, and Sulu would have warned them before they entered it.
Adam's songs are obviously dubbed. Not only is his singing voice much clearer than his speaking voice, with no echo or resonance from the set, it occasionally does not match his mouth. This is particularly obvious when he sings his duet with the hippie girl.
Not only is the "insane" Dr. Sevrin left with a single guard and no surveillance, but there are no guards and only one crewman in the room from which one can instantly and irrevocably take control of the entire ship! Previous takeovers have happened, but through Engineering after overcoming numerous crewman.
After the hippies take control of the Enterprise, they calculate three hours to Eden. Why does Kirk wait until they are actually in orbit around the planet before attempting to break into Auxiliary Control? Granted, Sevrin has threatened them if they attack -- but that would be just as true in orbit over Eden as it was when they were still in Federation space.
When the crew goes down the director used footage from "Spock's Brain" - you can tell because Chapel's hair-do changes to the style she had in that episode, rather then the one she wears the rest of this episode.
Shots of Kirk are also reversed when he walks out of sickbay after learning about Sevrin's disease.
When they find the dead hippie after they land, the actor is still moving his right hand repeatedly.
Kirk calls the bridge and asks if they have control of the ship - he gets the answer "No." Then a couple of seconds later he asks if they can break orbit and he's told "I think so." This seems contradictory.
For the first and only time in the series, Kirk just mentions "starbase" without qualifying it with a number. We never do find out what starbase they're close to in this episode.
When Kirk and company arrive on the planet Eden to investigate, in one scene the film is reversed and Kirk's insignia is on the wrong side of his uniform.
Adam: Stiff man puttin' my mind in jail, and the judge bang the gavel and say, "No bail!" Gonna lick his hand and wag my tail...
Hippies: Herbert! Herbert! Herbert! Herbert!
Spock: Many myths are based on truth.
Tongo: Tell Herbert it's no go!
Hippies: No go! No go!
Kirk: What does "Herbert" mean?
Spock: It is, um, um, somewhat, uh, uncomplimentary, Captain. Herbert was a minor official...notorious for his rigid and limited... patterns of thought.
Kirk: Well, I shall try to be less rigid in my thinking.
Adam: Gonna crack my knuckles and jump for joy! I got a clean bill of health from Dr. McCoy!
Kirk: Yes, Scotty, what is it?
Scotty: Captain, I just had to give one of...those barefooted what-do-you-call-ems the boot out of here.
Charles Napier's (Adam) acting debut.
The original draft of this episode was much different than what appeared on screen. The original script dealt with McCoy and his granddaughter, Joanna, with Kirk, not Chekov, falling in love with her. The script was heavily rewritten and the McCoy angle was dropped, but several novelists have used the initial idea for their books.
D.C. Fontana co-wrote the story for this episode under the pseudonymn Michael Richards.
The planet Eden is also mentioned in Star Trek V: The Final Frontier, and although Sybok was obviously thinking about a different Eden when he and his followers took over the Enterprise for much the same purpose as Dr. Sevrin and company, Kirk, perhaps remembering this adventure, stated many times in The Final Frontier that Eden, as a planet, is a myth.