- Dolled-Up Installment: Jerome Bixby based this episode very loosely on his own short story "One Way Street". In the original draft script, Kirk travelled to the parallel universe alone and the parallel universe Federation was battling a race called the Tharn. This name was later given to the leader of the Halkan Council, although it is not spoken on screen.
- Prop Recycling: The metal head sculpture in the mirror Spock's quarters (also in the room in "Amok Time") previously appeared in the possession of Dr. Adams and Marla McGivers.
- What Could Have Been:
- In Jerome Bixby's original outline, the Mirror Universe Federation was not evil, but simply backwards in terms of some technology, notably phaser weapons. Initially, mirror Kirk was married to a nurse on board the parallel Enterprise, and mirror Spock was more Vulcan in temperament. In addition, McCoy's counterpart, and not that of Spock, was to be bearded.
- In the original story outline, Captain Kirk was trapped in the Mirror Universe alone, and it was gradually rejecting him, treating him like he was an invading germ by poisoning his systems. Both ideas were dropped.
Trivia / Star Trek S2 E4 "Mirror, Mirror"