10 Times Star Trek Broke Its Own Rules

10. The Romulan Battlecruiser - The Enterprise Incident

The issue actually occurs due to production timelines. The Howard Anderson Company were tasked with the visual effects for both this episode and Elaan of Troyius, both of which feature the D7 Battlecruiser. This episode was written to tell the story of the Pueblo Incident, in which an American crew were taken prisoner and held for almost a year, accused of espionage, after wandering into North Korean waters.

Thus, this episode was aired first - the D7 Class a Romulan vessel, rather than a Klingon one, in terms of appearances. Spock does note that 'The Romulans are using Klingon designs now' - but this is something of a gaff in the schedule really!