Early drafts for Harlan Ellison's teleplay "City on the Edge of Forever" included a guest character, Beckwith, an Enterprise crew member who dealt in addictive "Jewels of Sound; it was Beckwith who escaped into the past, via the Guardian of Forever. Gene Roddenberry asked him to change this element, on the grounds that no member of *his* crew would ever use or deal in illegal drugs. According to Ellison's account in the book "Harlan Ellison's the City on the Edge of Forever: The Original Teleplay That Became the Classic Star Trek (1966) Episode", for years after the series was canceled, Roddenberry said that Ellison's original draft had been unusable because "he had Scotty dealing in interplanetary drugs" - although Mr. Scott does not even appear in that draft.
When William Shatner and Joan Collins are walking together on the street, they pass in front of a shop with the name Floyd's Barber Shop clearly painted on the window. This is the same Floyd's Barber Shop that is often seen on The Andy Griffith Show (1960), adjacent to the sheriff's office, in the town of Mayberry.
In Harlan Ellison's original story Beckwith's change of the past is revealed by members of the Enterprise team who are beamed back to the ship, only to find it is now a pirate vessel named The Condor. This is reminiscent of another episode Star Trek: Mirror, Mirror (1967) when Kirk and company beam into an alternate universe.
One of only two times in the original series that a "curse word" is heard, when Kirk says, "Let's get the hell out of here" at the very end. The second is in Star Trek: The Doomsday Machine (1967), when Kirk sees the Enterprise being drawn into combat with the Machine he says "[What] the hell's going on?".
Harlan Ellison's original story also described the architecture of the city of the time portal as "covered with strange runes". Somehow this was interpreted as intending for the city to be depicted as being covered by the remnants of architectural ruins when visualized for the filming on set.
When Kirk gazes upwards, the star pattern changes. This was mistaken as an error before the fade-out of Act One. The starscape effect was to visualize for audiences that no starships (at least from Starfleet) exist in the present.
Harlan Ellison's original script was extensively rewritten by D.C. Fontana at Gene Roddenberry's behest. Ellison was very unhappy about this, even though the episode won numerous awards and is regarded as one of the classics.
Just Kirk and Spock talk about the "flop", the scene changes to a street view, where a Kosher Meat store, with a big star of David, is conspicuously displayed in the center of the scene, this is one of the very few times a human (earth) religious symbol is displayed.