Although it was not intentional, Desilu's research department realized that Gene L. Coon's screenplay strongly resembled a novella of the same name by Fredric Brown. To deal with the difficulty, Brown was telephoned about the matter and he agreed to a writer's fee and an official credit for the story.
This is the first episode to:
- establish that the Enterprise's cruising speed was warp 6
- be directed by Joseph Pevney, brought in by producer Gene Coon. Pevney was known for his fast work, and finished this episode - originally expected to be shot in seven days (one day extra) - in six days, remaining on schedule, for which he received a $500 bonus. This comes out to $,4533.42 in 2022 adjusted for inflation
- establish the existence of a "Federation". The word was first used in The Corbomite Maneuver (1966) as First Federation, but it was the name of Balok's organization. This episode refers to the "Federation," which was fully named later in A Taste of Armageddon (1967).
- make a reference to "Star Fleet", when Kirk makes an entry into the recorder given him by the Metrons. He asks that "Whoever finds it, please get it to Star Fleet."
- be broadcast in color by the BBC, when it was first shown in the UK in November 1969
- establish that transporters cannot beam through deflector shields
William Shatner currently suffers from tinnitus due to an improperly timed special effects explosion on the set of this episode. Both Leonard Nimoy and DeForest Kelley reportedly suffered from tinnitus as well during the remainder of their lives.
Ted Cassidy (Lurch of La famille Addams (1964)) has his final Star Trek role as the voice of the Gorn. Cassidy had also voiced the antagonist in The Corbomite Maneuver (1966) and appeared as Ruk in What Are Little Girls Made Of? (1966), which aired in reverse order as compared to their filming dates.
The Gorn is not seen until 23 minutes in, almost halfway through the running time. Despite this alien's impressive debut, and the cultural popularity of the image, no Gorn was ever seen again in a mainstream Star Trek production until In a Mirror, Darkly, Part II (2005) almost 40 years later. (A Gorn did appear in The Time Trap (1973), which has sometimes been considered non-canonical.)