To some extent, this is par for the course in the Star Trek universe, especially with the original series.
- The Making Of Star Trek says the writers of Star Trek: The Original Series gave the Enterprise such a huge crew so they could introduce new characters who had apparently been there all along.
- We didn't know who Spock's parents were until they'd literally come onboard, we didn't know he had a half-brother until Star Trek V: The Final Frontier, or a sister until Star Trek: Discovery, and Sulu was never mentioned to have a daughter until we see her serving as the helmswoman on the Enterprise-B in Star Trek: Generations.
- Star Trek II: The Wrath of Khan:
- Khan meets Chekov and says he knows him because "I never forget a face." Which means that Chekov was on ship during "Space Seed," a first-season episode, despite the character not appearing on screen until the second season. Numerous explanations have been thrown about (he was on the ship, just not as bridge crew; Chekov was the officer in charge of delivering the supplies and food to Ceti Alpha V; an obvious joke about Chekov occupying a latrine Khan wanted to use), but ultimately it comes down to this trope.
- Another example from Wrath of Khan is Doctor Carol Marcus, Kirk's old flame. They even had a son together. McCoy's aware of her too (presumably from back in the day). And yet we're only hearing about her now, after all these years. Justified, however, because assuming David is the same age as his actor, Merritt Butrick, he would have been born, and Kirk and Carol's relationship would have been, several years before the original series, and Kirk explicitly states he "stayed away," as Carol wanted, after David was born.
- Some have speculated that Carol Marcus was the "blonde lab technician" that Gary Mitchell set Kirk up with, mentioned in the second TOS pilot "Where No Man Has Gone Before".
- Valeris in Star Trek VI: The Undiscovered Country, as a replacement for Saavik. The first drafts of the script did have Saavik in that part, but the writers ultimately decided that having Saavik betray the crew of the Enterprise and frame them for the assassination of Gorkon just didn't make sense for her character. (This is something that's been disputed by Nicholas Meyer. He wanted it to be Saavik, but Roddenberry leaned on him a bit because he didn't believe it'd fit the character. Meyer may still disagree, seeing as how he came up with Saavik to begin with.)
- Star Trek: Generations: Kirk's ultimate fantasy world in the Nexus involves the one true love of his life... who we've naturally never heard of before despite Yeoman Rand, Carol Marcus, or even Edith Keeler now being quite viable options for that role. In fairness, the original series made it plain that Kirk had plenty of old girlfriends, and he made quite a few new ones along the way (this is Kirk, after all). The real surprise is that David Marcus was his ONLY child. (Explained in the novelization of the movie; what Kirk actually did was experience similar fantasies with all of the women he'd truly been in love with.). They try to Hand Wave this, saying that Kirk retired from Starfleet at some point between Star Trek: The Motion Picture and Star Trek II: The Wrath of Khan, and returned to Starfleet once they broke up.
- The Cardassians are introduced in the The Next Generation season four episode "The Wounded," where it is explained that it has been only a year since the end of the long, costly war between the Federation and the Cardassian Union. However, this information means that the first two years of the show occurred during a war that was never seen, heard or experienced. Just where, exactly, was the flagship of Starfleet while the rest of the fleet was engaged in active operations? And how is it that Chief O'Brien is a veteran of the Cardassian War when he's been on the Enterprise-D since TNG's first episode? Was the Enterprise fighting in the war between episodes and nobody bothered to mention it, or are we supposed to assume that the first four seasons all took place in a single year despite all indications to the contrary? Admittedly there are a few hints that while to the Cardassians it was a major war, to the Federation it was a basically just a regional conflict, albeit an unusually long and bloody one for the period. If O'Brien served, presumably the conflict went back to before the Enterprise-D was commissioned and he was posted there. It's also possible that the first four seasons took place during a prolonged ceasefire leading up to the official peace treaty during which time the Federation was in a technical state of war with Cardassia. (There are actually countries in real life that are technically at war with each other despite not having fired a shot in decades).
- Even Ben Sisko has a bit of this, as he's introduced in the DS9 pilot as an officer on the Saratoga during the Battle of Wolf 359. This one's at least justified, since we never actually see the battle in "Best of Both Worlds", only its aftermath.
- Star Trek: Voyager had an episode Lyndsay Ballard, a crew member who had died and been resurrected by aliens, returns but no longer fits in; she had never been seen or mentioned before. This despite it being a Star Trek series, the Trope Namer and Trope Maker of the disposable one-shot crew member phenomenon. It's not like there's any lack of established dead or missing crew members to bring back. In fact, Ballard bears enough similarity to Ahni Jetal (see the "in-canon" examples) that it's likely that they couldn't get Jetal's actress back or something.
- Amusingly played with by the minor character Joseph Carey, who was a regular in the first season before disappearing from the show, except for flashbacks and time hops to the period of the first season. He then showed up again in the last season without comment, just in time to be killed off. A common fan theory is that the writers forgot they hadn't actually killed him off yet.
- Captain Picard remembers the Borg Queen when she's first introduced in Star Trek: First Contact. Apparently, she was there during the events of Best of Both Worlds, but Picard doesn't mention this vital piece of information about a dangerous enemy onscreen, and Data seemingly knows nothing about her despite the fact that he presumably would have read any report Picard made after the incident. They attempt to handwave this by implying that Picard had forgotten about her until they're reintroduced in the film.
- Star Trek Online: The mission "Storm Clouds Gather" had a generic Klingon NPC named Adet'pa whom the Player Character (all factions) interacted with for all of one dialogue chain. Two real-life years later, the character was given a new model and voice actor (Rekha Sharma) and upgraded to a recurrer for the Klingon Civil War story arc, and reintroduced as a hardcore loyalist of current Arc Villain J'ula. She addresses the Player Character as if they should automatically remember who she is, without explanation.
- The titular ship from the Star Trek: Enterprise prequel. The Next Generation would often refer to the Enterprise D as the fifth ship to bear that name, implying that The Original Series was the first. Enterprise also says there was an earlier Enterprise called the XCV 330.
- In both Star Trek Into Darkness and his debut episode Space Seed, each set in different continuities, Khan Noonien Singh is established to be a genetically/eugenically engineered Ubermenschen despot from the late 20th / early 21st century, the most prominent of several in fact, who ruled various nations across the globe, who partook in a destructive global conflict known as the Eugenics Wars. Despite this, nobody on the crew of the Enterprise has even heard his name (including the ship's historian), and they are only vaguely aware of what must have been a fairly major era in world history. Especially noticeable since they seem fairly knowledgable of several other historical figures, yet the guy who sounds like he was a latter-day Napoleon or Julius Caesar is a mystery to them. A possible justification could be that most knowledge of the Eugenics Wars was lost in World War III (which occurred not too long afterward), making Khan a somewhat obscure figure.
- Actually, in Space Seed Scotty mentions always having had a sneaking admiration for Khan, so he was known. It's just that up to that point he'd only identified himself as Khan and it's not likely they'd recognize him by face and first name alone. Only the actual historian was able to do that.
- Star Trek: Discovery is going this route as well, with the protagonist Michael Burnham being a human who was adopted by Spock's parents, yet Spock's step-sister has never been mentioned in any previous material. Played with in a rather clever way, since by the end of the pilot Michael's racked up a sizeable list of crimes, including mutiny and causing war to break out between the Federation and the Klingon Empire, and she's sentenced to life in prison; understandable that Spock wouldn't mention her. There's also the fact that Spock and Sarek weren't on speaking terms at the time, as Spock chose to enlist in Starfleet instead of going to the Vulcan Science Academy and then joining the Vulcan Expeditionary Group. Sarek also feels guilty for choosing Spock, when he was forced to pick either Spock or Burnham for a spot in the VEG, a choice that, in the end, was futile. Oh, and neither Burnham nor Spock mention Spock's half-brother Sybok. Later tied up in a neat little bow at the end of Season 2, with everything pertaining to Discovery and her crew, particularly the volatile topics of the spore drive, Time Travel, and Control, are declared Classified Information. Everyone with any knowledge of the events, including Spock, swears to Let Us Never Speak of This Again. It's also been noted that even without this, Spock's first canonical appearance with his family gives the strong impression that he and Sarek might have gone the entire mission not mentioning their relationship had Kirk not made an offhand comment about visiting "your folks on Vulcan."
