How about the, “Oh shit, this is a franchise! Let’s add more crap!” phenomenon?
Pitch Black was a perfectly serviceable horror flick, but the horrendously complicated backstory created for The Chronicles of Riddick would ruin it if you thought about it too hard. Likewise, The Matrix universe was overly complicated for the sequels.
A large part of what made the original Ringworld so great was the mystery: Here’s this artifact, clearly made, and nobody has the damndest clue by whom, how, or why. But in a universe with Protector, it’s obvious: It was made by Protectors, and you’d have to be brain-dead to not jump straight to that conclusion.
Okay I’m confused. The first movie is called Alien. The second is Aliens. Then there is Alien 3, Alien: Resurrection, and three more movies with another coming out this year.
I mean I like Alien and Aliens, have seen but hate Alien 3 and Alien Ressurrection (and have all 4 in a DVD box set that I got dirt cheap during the Great Online DVD Price War of the late 1990s-early2000s), and have never seen any of the “verses” movies or Prometheus, and have absolutely no desire to ever do so.
The Matrix was a gem of a movie, quirky, imaginative and new. Matrix 2 and 3 were progressively awful and made me begin to resent the first movie which occasioned them.
I can claim victory in this thread with Book of Shadows: Blair Witch 2. Not only did it destroy the premise of the original by making it “only a movie” within its universe, it forged ahead with a nonsensical “but spooky stuff anyway!” story. Supernatural stuff happens, just because. Couple that mess with crappy acting, crappy dialogue, cookie-cutter characters, and you have more than a crappy sequel; you have one of the worst movies ever made.
BW2 was so awful, one movie review site I used to love added a new basement to their rating system just for it. Previously, the worst was four bombs, described something like, “so godawful that it ruptured the very fabric of space-time with the force of its mediocrity.” The new worst rating was a mushroom cloud, with the description, “Proof that Jesus died in vain.” The review started with, “If you strapped a running camera to the back of a hyperactive cat with diarrhea and set it loose, the resulting footage would be better than Blair Witch 2.” I agree.
Okay, I’ll buy that. To my mind, an answer is a good thing. The first book was the “mystery” story, and the second et seq are the “answer” and “complications” stories. I think, if the mystery element had been continued, it would have become cloying.
(Instead, the “let’s move mountains with a reel of monomolecular superconducting wire and a stepping disk” plots became cloying. Ya can’t win!)
It could be worse, consider the three movies:
First Blood
Rambo: First Blood Part II
Rambo 3
Whatever happened to Rambo 2: First Blood Part III?
I could have maybe accepted an answer, but it had to be an answer worthy of the mystery. This is like Blue standing next to the door with a pawprint on the doorknob, the leash, and Steve’s shoes, and Steve needing a half-hour to figure out what Blue wants.
This is the only answer I’ve come up with. Even if a sequel is bad, the originals still exist. But Aliens 3 obviates every single thing that happens in Aliens in the first few minutes of the movie.
Except that the Protectors would never, ever build something like that.
Can we throw Clarke’s Rendevous with Rama in here? The first was a great space archaeological mystery with no real answers. The sequels tried to apply answers, and sucked.
I liked the second Matrix movie. However, the third one was bad enough to make me wish they had not made any sequels. Terrible. Certainly sours the original to me. Quentin Tarantino felt the same, I know.
Iron Man 2 is bad enough to almost made me never look at the original again. However, I thought Iron Man 3 is my favorite Marvel movie so far. Sometimes, things come back around.
Phantom Menace and Attack of the Clones are genuinely bad movies, though they do not ruin the originals. The third prequel was OK, but still not acceptable.
The Directors cut of Highlander 2 still makes no sense, by the way.
At the end of “The French Connection” it says that Charnier was never caught. In the sequel Popeye shoots him at the end of the movie. But I haven’t watched the sequel since it came out, so it almost doesn’t exist anymore.
Plus, they’re xenophobic to the point of neurosis. They would NEVER move alien species to Ringworld, they’d just exterminate them. The explanation in the sequel is poor handwaving - the Protectors couldn’t exterminate the species because it would be a giant red flag to Protectors in the area. But the Protectors wouldn’t care!
But, to the OP, it doesn’t ruin Ringworld - I simply ignore the sequels! 
(But Alien 3 - yep, that ruins 2, if you expect the full series to be taken as presented)
A similar thread from awhile back,* A love that will never die, until the sequel.
*Which was…only two years ago?! Jesus.
I still like “The Ringworld Engineers” (even though it doesn’t make any sense) more than “Escape From Hell”, Niven & Pournelle’s sequel to “Inferno”. Spoiler alert: Hell is full of ungrateful black people who tried to score freebies after Hurricane Katrina. Yeesh.
Watching Troll II made mre not want to see Troll.
There’s just no way Troll could avoid being a huge not-as-hilarious disappointment.
Sadly, this thread ruins that one.
I don’t see that Protector and Ringworld can’t exist in the same continuity.
Sure, scrith and twing are the same thing, but that could be just two independent people inventing/discovering the same material. Maybe the people that (really) built Ringworld reverse engineered the material from a protector ship. If you mean that the sequel “proves” Protectors built RW, then I agree with you. I never believed they did, even the first time I read RWE.
Protectors were seen as inhabiting the galactic core. They only rarely ventured out, so them ignoring the Ringworld is understandable. We know they came as far as Earth, but that’s 200 light years away from RW, and they didn’t have FTL drives.
The real problem wit RW is that no one would ever build it! It’s postulated that only a species with no other choice would need to build it. Plus, unless you’ve got some self-replicating nano-bot technology constructing it, the thing would take thousands of years to build, maybe tens of thousands. And if you have nano-bot tech (and scrith, and ramjets) you’d just go somewhere else rather than build that undefendable ego project, that blazing neon sign that says “come get us!”.
And the problem with later Niven is, when he decides he wants to change continuity, he simply says, “You know all that stuff you believe to be true, that people told you about Slavers/Ringworld/hyperspace monsters/Protectors? The stuff you’ve based you entire understanding of those things on? Well, the people that told you were lying!” That’s hack writing.
But I still love Ringworld. It would make a great movie, now that we have CGI good enough. So what if it has no real plot? Neither did Jurassic park. It’s just a fantastical escape story.