Because the original series was canceled instead of coming to a natural end, and the idea of a series finale was not as popular in the 1960s, "Turnabout Intruder" was closer to a normal episode and did not have the "finale" touches. The remastered version of "Turnabout Intruder" ended with the Enterprise flying toward a colorful nebula, to artistically signify the episode as being last of the TOS series.
William Shatner had a severe case of 'flu' during filming of this episode. At one point, he had to lift Sandra Smith in his arms, carry her to a couch and put her on it: during the first take, he got as far as the couch and dropped her. Fortunately it was well padded, and Smith bounced several times; according to Joanie Winston, who was visiting the set, Shatner looked down at Smith and said, "You know I love you, baby, but you've got to lose about six inches off that ass."
Leonard Nimoy is the only actor to appear in every episode of the series. William Shatner appeared in every episode with the exception of the first pilot, The Cage (1966).
Gene Roddenberry regretted the line about the Federation supposedly not allowing female captains, as he felt it was sexist.
The transposition sequence was the last footage shot for The Original Series.