Science fiction and fantasy television has produced plenty of bizarre or terrible episodes. But sometimes, an episode isn't just bad, it's world-breaking — because it introduces plot holes or character traits that would kill a show. Here are 10 episodes that everybody had to pretend just didn't happen. (Two of which are really, really good.)

1) Stargate SG-1, "Hathor"

This much-criticized episode introduced a lot of complicated rules about the alien Goa'uld that later got dropped or ignored, and the writers themselves sort of disowned it. The show even joked about it in a later episode: in "Heroes (Part One)," Dr. Janet Fraiser mentions "the whole Hathor incident, which we were never supposed to speak of again."

2) Red Dwarf, "Parallel Universe"

In the final episode of season two, the ship goes to a parallel universe where the characters' alternate selves are female. Naturally, the two Listers have unprotected sex, which results in the male Lister becoming pregnant — because in this universe, it's the men who bear the children.

So the season ends with Lister pregnant, and the writers intended to deal with that when they came back for season 3, but that episode never got filmed because it wasn't funny (and also maybe sexist.) Instead they lumped a whole bunch of plot points into a quick synopsis (Lister gave birth to two boys who did some advanced aging stuff and had to be returned to the alternate universe) and the pregnancy is only mentioned once, quite briefly, ever again.

3) Angel, "Eternity"

In this episode, Angel is drugged with a "euphoric" and briefly turns into Angelus… because apparently drugs are as good as true happiness (yay shortcuts!) A lot of people hated the episode (if you can just turn Angel evil with a pill, why don't his enemies do that all the time?) but Tim Minear has defended it:

I know there was a lot of criticism on the Internet about the way he went bad, and did he really go bad? The idea of giving Angel a variation of ecstasy—to me, that just worked. I thought it was a really good way to bring Angelus into the series for a moment so that he could interact with our characters for a moment, without doing some big 'Angel has turned evil' arc. You sort of get to have your cake and eat it too in that episode. It really works when he goes evil.

At the same time, Angel never took an "evil" pill again.

4) Star Trek: Voyager, "Threshold"

In this episode breaking the warp 10 barrier somehow results in Tom Paris and Captain Janeway becoming salamanders, running away to a swamp planet, mating, and having three salamander children. It was so bad that one of its writers called it "a royal, steaming, stinker." On the other hand, it won the 1996 Emmy for Outstanding Individual Achievement in Makeup for a Series, so that's something. Still, the notion that going faster than warp 10 will turn you into a salamander has never been discussed since, in any of the various iterations of Star Trek.

5) Buffy the Vampire Slayer, "Normal Again"

In this episode, Buffy is attacked by a demon whose venom causes her to believe that she's actually in a mental hospital and that the last six years have all been a hallucination… or, possibly, we discover that Buffy is in a mental hospital and that the entire show has taken place in her mind. The famous final scene, showing Buffy back in the mental hospital, leaves room for interpretations either way, and Joss Whedon has said that people can make up their own minds on the matter. For his part, though, he thinks that the events of the series really happened. The show never alluded to the "Buffy in a mental institution" scenario again, in any case.

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Update: Just to clarify this is one of two episodes on this list that we love, but which we feel was swept under the rug because of its implications for the rest of the series. Scroll down for the other one...

6) Star Trek: The Next Generation, "Force of Nature"

In this eco-inspired episode the crew learns that warp drive propulsion is actually tearing at the fabric of space, and the Federation decrees that all their ships must stick to a speed limit of warp five, except in cases of emergency. The speed limit is then referenced a few episodes later, only so that it can be overridden, and then… it just sort of disappears. An unpublished Voyager technical manual suggests that the problem may have been solved with "variable geometry pylons," for whatever that's worth.

7) Doctor Who, "Trial of a Time Lord"

This is the season-long Doctor Who storyline that ended Colin Baker's tenure as the Doctor (on television, at least) after the show was put on hiatus and nearly cancelled in 1985. In "Trial," the Doctor discovers that his evil side will come out in between his 12th and 13th lives, and will then go to Time Lord law school in order to become a prosecutor in the Gallifreyan legal system, so he can put his past self on trial. For a long time afterwards, the BBC reportedly had a prohibition on mentioning this story, but last year the Valeyard was namechecked during "Name of the Doctor." At the same time, neither John Hurt nor Matt Smith turned into an evil attorney.

8) Battlestar Galactica, "Black Market"

This is the episode where Lee Adama is dating a sex worker and taking part in a whole black market thingy, and at the end of the episode there's a scene where it feels like Bill Adama and his son sort of look at each other and say, "This never happened." And then... it's never mentioned again. Ronald D. Moore has been very vocal about disliking this episode, and its take on Lee Adama was basically ignored afterwards.

9) Game of Thrones, "Breaker of Chains"

The only part of this episode that everybody agreed to pretend never happened is the bit where Jaime Lannister rapes his sister Cersei, next to the tomb of their newly dead son Joffrey. There's actually an article in The Daily Beast with the headline "Why We Should Pretend the 'Game of Thrones' Rape Scene Never Happened." The show basically depends on everybody going along with this, because Jaime and Cersei have a number of interactions in the following episodes which make no sense if you see them as conversations between a rape victim and her rapist.

10) Doctor Who, "Father's Day"

We could also mention the Who TV movie where it's "revealed" that the Doctor is half-human. But the whole plot of this actually quite wonderful episode depends on the idea that if you cross your own timeline, huge time vultures will appear and eat everybody. If the show had stuck to that rule since then, a lot of the past five years would look very different, including the "Christmas Carol" episode but also a lot of Matt Smith's most pivotal episodes. The absence of time vultures showing up and devouring people every time they intersect with their own pasts is an abiding mystery.

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Update: Paul Cornell, writer of the episode, has taken issue with our take on it on Twitter, arguing that the show actually built on it. Relevant bits below: