This episode is the first to feature a female starship commander.
First broadcast episode of TOS to feature the D7 Klingon battle cruisers. Although the episode Elaan of Troyius (1968) was produced three months before this episode, and technically the first to feature the D7s, this episode was aired on television first, since NBC changed the airing order for all the episodes. For the Remastered series in 2006, digital shots of the D7s were inserted into scenes in the episode Errand of Mercy (1967), which now officially makes that episode to be the first to have the D7s.
D.C. Fontana's initial inspiration for this story and its title was the Pueblo incident which involved the capture of an American intelligence gathering ship, the USS Pueblo (AGER-2), by North Korean forces during the Vietnam War. North Korea claimed, without evidence, the ship had violated its territorial waters. The incident occurred on January 23, 1968, just two months before Fontana completed her first draft story outline. Although the crew was released after nearly a year, NK still maintains possession of the vessel as a "war trophy".
The Romulan cloaking device prop was created using the Sargon globe (Return to Tomorrow (1968)) and portions of the Nomad probe (The Changeling (1967)).
In the opening segment, in regard to the enemy vessels, Spock declares "Romulans now using Klingon design!" Several explanations/theories have been advanced as to why the Romulans use Klingon ships. The real reason is simply because the original Romulan model ship was gone. Non-union, independent creative designer Wah Chang designed and built the original Romulan Bird of Prey model for Balance of Terror (1966). He is also the creator of several iconic Trek costumes and props, including the communicator, original phaser, salt vampires, and tribbles, to name a few of his contributions to the show.
After "Balance of Terror", the model Bird of Prey was returned to Chang, per his contract, but he eventually disposed of it as he had limited storage and there was never any contractual obligations with CBS to retain it. However, either through poor planning or poor communication or both, season three's "Enterprise Incident" production staff, expecting to re-use the season one Romulan model, learned from Chang that it was gone. The decision was made to modify the story to use only Klingon model ships, which were kept by the studio, although in recent airings one of the three ships appears to be the original Romulan model. Asking Chang to construct a new Romulan model ship would have been prohibitively costly. Economically, switching to Klingon vessels was the obvious solution.
After "Balance of Terror", the model Bird of Prey was returned to Chang, per his contract, but he eventually disposed of it as he had limited storage and there was never any contractual obligations with CBS to retain it. However, either through poor planning or poor communication or both, season three's "Enterprise Incident" production staff, expecting to re-use the season one Romulan model, learned from Chang that it was gone. The decision was made to modify the story to use only Klingon model ships, which were kept by the studio, although in recent airings one of the three ships appears to be the original Romulan model. Asking Chang to construct a new Romulan model ship would have been prohibitively costly. Economically, switching to Klingon vessels was the obvious solution.