Let’s not forget Plan 9 from Outer Space. The mother of all noncontinuity.
The film Batman and Robin. Instead of Barbara Gordon becoming Batgirl, director Joel Schumacher introduces Alfred Pennyworth’s niece and has her assume the Batgirl identity. WTF?
The Weis and Hickman Dragonlance novel “Dragons of Summer Flame,” and then their next trilogy of books (The War of Souls.)
In “Summer Flame,” the Creator of the world/father of the Gods “Chaos” returns from being trapped inside a magical gem and wreaks havoc. In the end, one of the main characters from their first set of novels, who was pretty much equally loved and hated by fans, sacrifices his life to defeat Chaos (he gets crushed to death by Chaos, but in doing so, managed to barely slice him with a dagger, which was key to defeating him.) A few pages later in the book, another god appears to another main character and says that this latest incident was proof that the gods of this world aren’t worthy to have it, because they keep interfering, rather than sitting back and watching, which was their original goal when they created it. So he says they all agreed to leave to another plane of existence, or something, and not return for a long, LONG time, of at all.
In their next trilogy, they not only brought BACK the dead character (explaining it away as saying he used a magical time traveling device he had just before he died, which he did have, bit had nowhere near enough time to use,) but also said that they gods didn’t leave, but instead the world was “stolen” way from them so they couldn’t find it. (An evil god took the world away so that she would be the only god and the only one worshiped.)
Basically, from what I understand, the writers were “forced” into writing “Summer Flame” and in doing so had to provide a reason for the new game mechanics system that TSR (who owned the line of books and the D&D game world they were based on) was selling.
In the movie “First Knight” with Sean Connery and Richard Gere, Gere is fighting the Bad Guy in an epic duel to the death. He knocks Bad Guy’s sword out of his hand, the camera zooms to the weapon swinging through the air in several glorious arcs, it lands point first in the ground buried halfway to the hilt, signaling the impending doom of Bad Guy. Camera zooms back to Gere, ready to deliver the killing blow. Bad Guy is holding his fucking sword (WTF!) as Gere stabs him through the chest.
Few continuity errors can top the two flubs in the old westerns I mention in this thread.
Both involve characters that literally disappear from one scene to the next. The first example has three actors climbing a set of stairs, but only two emerge at the top! The second has three actors walking through a door, but, again, only two come through the other side! Those old B westerns were so pitiful. How did people watch such junk?
(Btw, I never did learn the name of the first film. If anyone knows it, I’d love to hear about it on this thread. Thanks.)
Oh my God! They killed Kenny!
That was actually (sort of…) explained away. Kenny’s parents have had DOZENS of kids, and they were all named Kenny, in honor of their son Kenny’s recent death. 
When the soap “Another World” first came on, Alice & Pat Matthews had a younger brother Russ who was a high school dropout. He ran away and came back two years later as a doctor who was older than his sisters.
That’s some great magic trick!
Oh, geez…if we start with soaps we’ll be here forever. Most of them have decades-long epidemics of SORAS (Soap Opera Rapid Aging Syndrome). The problem being, of course, that children are boring. About the only drama possibilities that are acceptable to inflict on children in television are kidnapping and serious disease.
Once they’re teenagers, then you can start in with all the myriad facets of drama connected with love, rivalry, greed and sex. So naturally the temptation is to just grow 'em up quick.
Or it was Marge playing an April Fool’s prank on Bart and making him go to church extra. 
“The guy was killed in an auto accident! I looked it up! He was driving in the Yukon, in a pink convertible, to visit his brother who’s an ex-con named Frances, when a tractor trailer comes along and decapitates him. You know what that mean, it means he doesn’t have a head. How am I suppose to write for a guy who doesn’t have a head? He’s got no lips, no vocal cords. What do you want me to do?” --Whoopi Goldberg as the soap opera writer in Soapdish
The absolute WORST continuity error in the history of The Simpsons was in the episode And Maggie Makes Three, where Marge is explaining that Homer lost his hair by pulling it out and running upstairs when Marge tells him that she’s pregnant. It shows flashbacks to Homer doing the same thing when he hears about Lisa and Bart, even though during both flashbacks, they are shown in their current house (they didn’t move in there until after they both knew that Marge was pregnant with Lisa) AND there are photos of present-age Bart & Lisa on the walls! Also nevermind the fact that other episodes already showed Homer finding out about Bart and Lisa, in completely different situations.
I think the most egregious continuity error in Simpsons history is the episode in which Homer escapes a duel by running away to the house he grew up in…despite the fact he burned that house down in an earlier episode.
“Any time you see something like that happen, a wizard did it.”- Lucy Lawless (not Xena)
I’m not saying that Wikipedia is 100% correct on everything it says, but I thought that the commentary for that episode (I’ll have to hear it again to check) said the episode was inspired by various cases of soldiers returning to someone eles’s home claiming to be fallen comrades they befriended and taking over their live, as well as a film called Somersby with the same premise (the episode was originally to be called “Skinnersby”). One of the staffers also comments that even though he thought the idea for an episode was a good one (and I like it, too), he can see why the fans were angry for claiming someone we all know and love really isn’t who he says he is, giving as an example how he felt when a movie based on a popular TV show (I won’t say which one) revealed that a well-known character was actually an impostor claiming to be that person who he really isn’t.
The movie Sommersby is based on a French movie called The Return of Martin Guerre which is based on a real incident in France in the Middle Ages when a soldier returned from a war, just enough older that no one made a big deal about how he looked a little different. In fact, he was a comrade of the soldier (who looked rather like him) who decided to take over the missing soldier’s place in the village and in the solder’s family. Eventually, the villagers became suspicious and it was discovered that the real man was still alive but hadn’t returned to the village. The comrade who pretended to be him was executed.
Now I know David Eddings is a souless hack but in early Jr High I enjoyed his Belgarad series. Later on I saw he did a few sequels that I read one called Belgarath the Sorcerer IIRC that flatly contradicted several plot points that had been raised before. For instance it’s revealed to be a VERY big deal in one of the books if someone uses sorcery without guidance. So much so they stop to chat with a person to try to figure out how he did it. During this they cover how Beldin used the power for the first time to destroy a rock instead of attacking Belgarath who he was furious with. In Belgarath the Sorcerer (which was a prequel) the first time Beldin and Belgarath meet Beldin uses sorcery reveals he had been for a long time and really doesn’t think it’s a big deal. Not only was that a tedious retread of a book (which BTW came after he had already done a series that was a retread of the original series) but was pissing on the rules he had already hedged and rearranged at his whim.
My problem with that episode isn’t the premise but the fact they really didn’t do enough with it once the “real” Seymour Skinner that was voiced by Martin Sheen took the place of the “phony” Skinner’s (a.k.a., Armin Tamzarian). I thought it would’ve been funnier if after being exposed to a few days of the general stupidity, incompetency, and insanity that is life in Springfield, he’d get so frustrated that he’d go to Tamzarian and tell him, “You wanna be Seymour Skinner? Go ahead! * I’ll * be Armin Tamzarian!” The last scene then would’ve been the “real” Skinner riding off in Tamzarian’s motorcycle as the phony “Skinner” returns to his old life. (Of course, that’s just me being an armchair scriptwriter again.)
That would’ve been a little too much like what happened to that Frank guy, IMO (“normal” person meets Springfield, gets frustrated with it). Besides, he DID apparently live and grow up there before going to 'Nam, so he would’ve known what it’s like beforehand.
True, but he’d been gone for 30 years so he could’ve gotten acclimated to the “outside” world and forgotten what it was like. While it is true he spent much of that time as a POW, he could’ve said something that even the hell of that existence was still infinitely preferable to living a few weeks in his old home town (just having to deal with his mother Agnes would’ve been too much for him to bear).
Incidentally, was Tamzarian ep before or after the Frank ep? I’ve forgotten which one was aired first.
The new incarnation of “Doctor Who”.
I love the show, but this bugged me. In the two part finale of season two, The Doctor repeatedly hammers home the point that the Void (i.e. the space between alternate realities) is made of nothing. Nothing. No matter, no time, no energy, no nothing. Later, in the same damn episode, he successfully throws the Cybermen and the Darleks into the void using the “Void Stuff” that they accumulated during their travels through the Void. How can absolute nothingness have “stuff”?
How is that discontinuous with any of the previous movies?
As for in-movie continuity errors, I think the all time champion is the sequence in “The Jagged Edge” wherein Glenn Close wears three different suits during the same scene. She enters the courtroom wearing a grey suit, then addresses the jury wearing a dark black or blue suit, and then when she sits down she’s suddenly wearing a brown suit.