To emphasize on the extremely high age of the Guardian in the upper millions, or well into the billions, the starfield of its planet is surrounded by red dwarfs and red giants.
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 which 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 idea was later used in Mirror, Mirror (1967).
Widely considered by both fans and critics to be the best episode of the series.
Gene Roddenberry apparently denied Harlan Ellison's pseudonym request because he knew everyone in the science fiction community was aware that the "Cordwainer Bird" credit was Ellison's way of signaling his dissatisfaction with the way production people treated what he wrote. It would have meant that Star Trek was no different than all the other "science fiction" shows in mistreating quality writers, and could have resulted in prose science fiction writers avoiding contributing to the program.