10 Times Star Trek Broke Its Own Rules
With so many years behind it, Star Trek was naturally going to face canon issues from time to time.
As it stands, all televised series of Star Trek are considered canon (more on this later). There's an issue that comes up straight away, in the remastered edition of the Original Series. Designs change, along with entirely new ships added. This is an example of adding to the existing canon, while technically breaking the established rules.
Canon is a hard pill to swallow in Star Trek. Including the above reference, there are so many different examples of things changing throughout the franchise. Gene Roddenberry was selective when it came to what was and wasn't locked down lore. He initially didn't count there to be an ongoing continuity between the Original Series and The Next Generation (not to mention the Animated Series), meaning that rules could simply change as he saw fit.
While many examples of canon violations in the early years of Star Trek can be explained away as Roddenberry simply enjoying himself, later canon violations become somewhat harder to explain away if the show is set on keeping continuity.
Looking at you, Star Trek: Discovery.
Let us take a few examples of the times when canon issues have arisen in the course of the franchise.
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!