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."
"Turnabout Intruder" which is the final episode of The Original Series, takes place in 2269, 4 years before Star Trek: The Motion Picture (1979) which took place in 2273, even though they were released 10 years apart.
According to Harry Landers, he was very fatigued during this episode because he had just had his upper right lung removed due to an infection. He wanted to turn the role down but did it as a favor to producer Fred Freiberger.
In Roman mythology, Janus was a 2-faced head representing opposite aspects in nature such as chaotic nature and controlled civilization. "Lester" means in Old English: locality. Hence, Janice Lester. A split personality from nearby or within.
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.
After two years on the series, Roger Holloway finally gets to speak dialogue, all of two words. His character's name (Lemli) was the same as William Shatner's license plate at the time, a mixture of his daughters' (Leslie, Melanie, Lisabeth) names.
Lieutenant Galloway reappears in this episode, despite being killed by Ronald Tracey in "The Omega Glory". He was credited as Galloway (misspelled as "Galoway") even though actor David L. Ross had been recast as Lieutenant Johnson in "Day of the Dove" after the character of Galloway was killed off.
This episode is subtly referenced in the TNG episode "Legacy". In it, Jean-Luc Picard mentions that they are bypassing an archaeological survey on Camus II, the same planet that this episode begins on. This was mentioned because, with the airing of its 80th episode "Legacy", Star Trek: The Next Generation officially became longer than TOS.