In his book, William Shatner referred to this episode as "one of the worst." Leonard Nimoy has stated that he was embarrassed during the entire shooting of this episode.
Written by Lee Cronin, the pseudonym of Gene L. Coon. Some have assumed that it was used because he was unhappy with the results. Actually, it was because he had left Paramount and was under contract with Universal, so he was not supposed to be working for Paramount as well.
This is the only time moving stars are shown on the bridge view screen via rear-projection. This is evident because the camera pans across the screen and people move in front it with stars moving, which would not have been possible if the view screen scenery had been matted in later. (In The Doomsday Machine (1967) they walk in front of the screen, but it is a static picture of stars, although in the digitally mastered edition they did add some moving objects.)
First appearance of Scotty's new hair style, where it was brushed back instead of parted to the side.
In informal surveys taken at science fiction conventions, this episode is promptly and almost universally named as the worst of the original series.