In short, anything not G1. To some extent, internal strife is seen as part of the fun.
Even the almighty G1 falls prey to this, since there's fans of both the cartoon and comic continuities, and in more recent rimes, the G1 cartoon has built up a hatedom of its own — ironically, some fans have started hating on it only to "fight back" the purists who had originally started this "ruined FOREVER" craze. Hell, to some people, the existence of the franchise in general; even before all the internal hate began.
Callbacks to G1 in newer series have become a very frowned-upon thing, because in many fans' eyes, it means the show is sacrificing originality in favor of trying to please older fans. This also supposedly ruins classic references forever. Then again, Optimus' famous "One shall stand..." line has been used so many times in modern media, it has lost basically all its impact.
The reception to Transformers: Robots in Disguise is seemingly less vitriolic, possibly a sign of TF fans tiring of this trope. That said, haters of TF: Prime hate it because it's a follow-up of that show, and TF: Prime fans hate it because it's set to be a drastic departure in both tone and animation style. Essentially, "Bayformers Animated... Animated". And beyond that, the revelation of things like a non-G1-coloured Grimlock who might be a former Decepticon, animal-themed 'Cons who are not legacy characters, and Bumblebee as Autobot leader have caused all manner of "Ruined FOREVER" reactions among some fans.
Haters of the show frequently have this reaction when ponies are added to things they enjoy watching or listening to (such as in the case of PMVs, or remade anime intros featuring ponies, etc.), especially on YouTube, surprise, surprise. Usually their argument for why the show is terrible and why they hate all the fans of it is that it's "ruining the Internet", or "ruining" the medium the ponies have been applied to, even though it was them who clicked the link and watched the ponified media in the first place. And yes, this does lead to many a Flame War.
Episode 12, "Call of the Cutie", drew particular fire because it focuses on Apple Bloom, who had been a side-character up to that point. Some also were upset about sweet, kindly Chantal StrandPlaying Against Type as the Alpha Bitch Diamond Tiara.
Much of the hate seems to have stemmed from fears that the new series was starting to head the way the old one(s) did, with many episodes that revolve around a new (or bit-part) character who appears with a problem, has it solved, and then vanishes never to be seen (at least, outside the background) again. In fact, the episode served to introduce the Cutie Mark Crusaders Power Trio, who have since become a second set of main characters who are loved nearly as much as the first, and none of their subsequent focus episodes have drawn nearly as much ire.
Episode 23, "The Cutie Mark Chronicles": Rainbow Dash's first "Sonic Rainboom" is revealed to have led directly to all of the main characters earning their Cutie Marks, thus unknowingly ensuring that the future played out the way it did. That makes her too important/powerful/special, it's like those self-inserts in fan fiction.
Episode 26, "The Best Night Ever": Fluttershy, the shyest, quietest, KINDEST pony goes on a psychotic rampage? That's way out of character! What were they thinking?
It was clearly done Just for Fun, and up to that point everyone else in the cast had at least one trip over the comedic edge of sanity. And, really, it was hardly the first time Fluttershy got aggressive — it was just uncalled for in this case. In season 2, that theme came back as Fluttershy's self-help course in assertiveness sent her way overboard to angrily aggressive.
"Lesson Zero" got a lot of this. Within a half hour of the show being aired, there were people online saying the show had started "pandering" to the brony fanbase, and that the creators were "forced" to make episodes like this, with many even flocking to Lauren Faust's DeviantArt account to apologize for what the fans had done to her series.
To wit: Twilight slowly going crazy at the prospect of failing the princess, Rarity being a drama queen, Rainbow Dash wearing Cool Shades, and Big Macintosh saying "eeyup" and "nnnope" a lot. Cries of Flanderization ensued.
"The Cutie Pox" got labelled Ruined FOREVER because it had a minor Shout-Out to The Big Lebowski (a decidedly un-kid appropriate movie), and because it showed Apple Bloom immediately developing Aesop Amnesia. Despite this being a basic character trait of the CMCs in all of their episodes. Then again, that trait wasn't very well liked, either.
Post-"A Canterlot Wedding", a portion of those whose headcanon insists on immortal god princesses with infinite power were mortified at how easily Celestia was taken down by what amounts to a fairy queen. Not to mention the very existence of Cadance, the Pretty Pink Princess of Babysitting, and Twilight's brother.
The speculation that the Rainbow Power featured in some of the newer toys might be in the box from the Tree of Harmony already has people freaking out.
In 2014 Hasbro announced that they expected G4 to continue for ANOTHER FIVE YEARS. It wasn't long before cries of this were heard.
Anthro fanart - despite the original characters technically being anthropomorphic, and them being in a cartoon where anthro characters belong - specifically art where a character is given more human-like characteristics than the actual show, brings about the "RUINED FOREVER!" cries. Many anti-furries in the fandom despise the mere knowledge of furry artists being fans of the show; this leads to the occasional flame war and "RUINED FOREVER" reaction.
The term "Bronies" and the fan name, DJ PON-3 (even pronounced "DJ Pon-three"), being mentioned and canonized respectively in the 2011 "Equestria Girls" promo (the Katy Perry parody, not to be confused with the Equestria Girls movie) was met with concerns over fanservice getting out of hand. And that was before season 2 already.
The news of Derpy Hooves having planned appearances in season 2's episodes, rather than being a randomly thrown-in background character, led to some fans crying foul over the fanservice.
The store chain Hot Topic recently selling G4 T-shirts, due to the reputation of the store, and its average customers, current members of the fandom feel that the show and the fandom will be "RUINED FOREVER" by the publicity gained through that store.
Rule 34 art exists, but good luck finding a fan who will openly admit to enjoying them.
The flame war over Derpy's name and voice being changed in all further airings of "The Last Roundup" is a mix of They Changed It, Now It Sucks and this trope. Even when the Studio B staff said explicitly that it's not the end of her character as she was previously portrayed.
Derpy returned in "Rainbow Falls", wall-eyed and all, and was even made a spare member of the Ponyville team. Some Bronies raged about the Fanservice that had gotten out of hoof so much that it finally ruined the show FOREVER (if it hadn't done that before already), some Bronies complained loudly that Derpy didn't speak as though she was still censored. Since one hardly ever hears the many pleased Bronies cheer in between the raging ones, the fanbase appears more and more to be unpleasable.
MLP canon that is not MLP fanon gets this reaction rather often. As an example, there was an entire Flame War because Heartstrings's name was not one of the many names the bronies wanted Hasbro to pick. Hasbro eventually gave in, added her old Fan Nickname to her toy name (she has been Lyra Heartstrings ever since) and seem to look up Fan Nicknames now whenever they have to give a name to a toy based on a background pony.
Related to the above, when Lauren Faust announced that she was working on a new My Little Pony show, the Internet exploded with rabid fans of The Powerpuff Girls and Foster's Home for Imaginary Friends who were furious that Lauren had begun working on "a glorified toy commercial", called her a sell-out, and that her entire ouvre was RUINED FOREVER!!!1
G.I. Joe seems to be undergoing its own version of this right now. While one of the smaller hardcore fanbases out there, the vast majority of said fanbase seems adamantly determined to apply this trope to the live-action movie. Primarily because it dares to suggest that GI Joe might not strictly be a Real American Hero.
More like, "The Baroness was only brainwashed? DEATH OF THE LINE!" Now, this was an actual out-of-left-field plotline in the original comic run, but she found she quite enjoyed being a bad girl and was already shacking up with Destro. So the real crux of many arguments falls to by turning her into a goodie it eliminated the excuse of the nostalgia filter hot-as-all-getout leather-clad Baroness in subsequent movies due to her normal personality being a delicate wallflower. Thankfully, it doesn't seem like that particular storyline will go anywhere since the movie flopped. Also, the final climatic underwater battle actually came off as less realistic than even the cartoon, which had a no-kill policy! It was just a poorly handled movie overall.
The Joes also had their fair share of wackjobs in the original, but the big anger over characterization is that they had an offensive as Michael Richards Hollywood Jive Negro as the sidekick, when the original had Roadblock, who still remains among one of the most positive portrayals of a black guy ever in a children's cartoon (not an easy shoe to fill), and also put Duke back into being a new recruit, despite all other characters being already active and accounted for. Many many problems with the movie come about because the director seems to have not done the research, even though he did, he just put too much Hollywood into the franchise. The running time doesn't leave much room to flesh out anybody either. The animated movie which aired on CN got a MUCH more positive reaction. (Aside from Cobra Commander's rant, which is strangely based upon the fact he used the word crap.)
The G.I. Joe: Sigma 6 cartoon was under a considerable amount of flak by the fanbase as well, because of the use of a Japanese animation studio (GONZO). Comments such as "The Real American Hero shouldn't be an anime!!!" were common during its run.
To be honest, animation wasn't all that they used...The very worst elements of anime were in full effect. Its death was assured the moment it aired, as the target audience were NOT anime fans. (and to the anime fans it did attract, the toys were not up to snuff, as they are far more experienced with the high-quality static resin figures.)
Also, the first series was also done in Japan (by Toei Animation) as well.
The humorous "DETH OF TEH LINE!!1" originated from overreaction to changes in how the G.I. Joe franchise was being handled some time in 2005, is regularly used by some G.I. Joe online forums when some react negatively to new releases or change of plans. It's sort of the Joe equivalent of "RUINED FOREVER".
Static Shock and the "Hoop Squad". Were the super-powered basketball players absurdly dumb? Yes, of course they were. Did their appearance mark the show's all time lowest point? Almost unanimously. Was it one of the worst uses of guest-stars ever? Quite easily. Did the show actually get any worse after that? Not really.
As soon as it was announced, The Fairly OddParents!'s fandom was torn over whether or not Poof's birth was yet another shark to jump. Most believed that the show was Ruined FOREVER.
Then there's The Movie. Which is live-action, and involves a 23-year-old Timmy still in primary school with his fairy godparents with CG so bad that it will make the live action Yogi Bear movie look like something that came out of Pixar. What.
The third FOP movie is the WORST. They had Timmy turn into a fairy at the end!
And now there's Sparky.
And now there's Chloe.
Strawberry Shortcake has been prone to this due to the uncertainty of the direction each of the master licensees want to take the franchise which has angered fans due to its frequent changes. When the 2003 version started, some of the older fans felt that the franchise was doomed. And when the 2003 version was aged-up in 2007, there was trepidation from some (particularly, fans of Honey Pie Pony, who weren't pleased that she had retired to Ice Cream Island permanently, and even then the send-off was only mentioned in a chapter book and not on the TV series officially, and after that the existing fillies, Honey Pie and Ice Cream Island was never mentioned ever again, while others who disliked the Pie-Man voiced their dislikes in the franchise bringing him back). Then there's the 2009 series, which were widely decried by the educational TV connisseurs who're drawn to the series by the 2003 series being educationally wholesome as lacking in terms of educational content and emphasis on beauty over moral values, the transition to CGI, as well as being nothing more than a 30-minute toy commercial.
Another American Greetings franchise, The Care Bears, went through this and are going through this again. Some people decried the franchise as ruined forever when they redesigned the bears in 2005, and now they're going through another round of fire when plans leaked that they're going to revert back to the pre-2005 designs and fans aren't pleased.
An ironic form would be The Spectacular Spider-Man, people were angry about the style, mainly the fans of the 90s show and comics, who thought the show would be kid-oriented, but after watching a few episodes, it became very well thought of by comic book fans, but some will still try to discredit the show.
Teenage Mutant Ninja Turtles went a very specific instant of this. As soon as the 2003 animated series was released, fans of the '80s cartoon complained because it was so drastically different in tone to what they had watched 20 years prior (dark, stylish and character driven as opposed to outright cheesy, goofy and with little motive to enhance the plot beyond a single episode). Thus, complaints were made to bring back the ORIGINAL Turtles. The irony of this, is that the 2003 series was based on the original Mirage comics that started from 1984, as opposed to be much more lighthearted cartoon that first aired in 1987 and that one of the Turtles' co-creators (Peter Laird) was actually involved in the production of the 2003 series.
Of course, partly in response to this the series was retooled into TMNT Fast Forward where the Turtles are transported 150 years into the future; the series was dumbed-down and made more kid-friendly. The poor response to that change led to the following season (which turned out to be the final one) seeing the Turtles returned to the present-day, and darkened yet again.
Also, similar to the Disney/Marvel example above, Peter Laird selling the TMNT franchise to Nickelodeon.
Laird had his own "RUINED FOREVER!" moment, apparently, over the character of Venus de Milo in The Next Mutation. The moment he bought total control over the property, he ordered any and all mention of her in every other continuity of the media excised without in-story explanation, with the other characters seeming not to notice. The character was such a Berserk Button for him that anyone coming on board the creative staff was advised to never even say the name around Laird, even as a joke.
Michael Bay is going to be making a TMNT movie? Well, his Transformers movies were hit or miss, but he still did okay...the turtles are going to be aliens from space instead of normal turtles mutated by ooze? BAAAAAAAAY!!!note This turned out to not be true in the end, being based on a rejected early script optioned to Bay, who turned it down. A scene was added into the movie to make fun of it. Either way, a lot of fans believed this or still do even after the movie came out and it turns out it is, indeed, ooze that mutated them.
Speaking of which, both the 2003 and 2012 cartoons have gotten flak from some camps for pulling the "character is an alien" twist in each of their second seasons, namely Shredder in the former and April in the latter.
People are preaching this about Iron Man: Armored Adventures solely because Tony is a teenager in this adaptation. Granted, perhaps they do remember the actual teen Tony from the comics...
All-Stars.... Quite a few are even going as far as to say that because of this season they'll either be quitting the show for good after Pahkitew Island (since it'll temporarily eliminate the problems from that season due to having another new cast) or won't even bother giving that one a chance at all and just tossing in the towel now.
EVERY SINGLE EPISODE of The Simpsons has had this accusation hurled at it by one fan or another. Go back in time to the ancient days of usenet and you'll find post after post declaring each and every episode the "worst episode ever" (the phrase actually originated on a Simpsons forum) and destroyer of the franchise. This even applies to the very first episode, which some fans of the Ullman shorts hated. Today, no matter how good or bad an episode is, you can bet someone somewhere on the internet will make an overdramatic "this is the end of The Simpsons" post about it.
Other things which have ruined the Simpsons forever include: the movie, the Butterfinger commercials, the Coca Cola Super Bowl commercial, the Tik Tok music video in the opening, and anytime Matt Groening says anything to the press.
A reviewer at The AV Club opened one review with an essay about how the Frank Grimes episode, despite being a good episode, ruined the Simpsons forever.
A frequent complaint is that the "panda rape" episode ruined Homer forever (Homer put on a panda costume and was accosted by a smitten male panda ... which is nasty if you really think about it, but is also what happens to EVERY character who puts on an animal costume in a slapstick comedy and then encounters a real version of that type of animal). As if it somehow made Homer "dirty."
The Armin Tamzarian/Principal Skinner fiasco deserves special mention here. This is most frequently cited as the reason the "Golden Age" ended.
Funny enough, on the 500th episode, the crew actually appeals to the fans to, "get some fresh air before you go online and complain about how much this episode stinks", showing that they are aware of how divisive their show is.
Western Animation in general has always had someone declaring it Ruined Forever. Winsor McKay declared the medium ruined forever when animation studios started using cels, it was declared ruined forever when it moved from movie theaters to television (not entirely without reason though), and now with the advent of computer animation, crudely drawn Thick-Line Animation and the influence of anime, it's once again considered ruined forever by many.
South Park is pretty much ruined forever in the eyes of the fans thanks to the episodes "You're Getting Old" and "Ass Burgers" because the idea of Stan Marsh becoming more depressed and cynical has been seen by disgruntled fans and ex-fans alike as Trey Parker and Matt Stone having raped the character and ruined what it means to be a child.
"Butterballs" also went down this way in some circles, as even if Butters' plot came to a reasonable close, the rest of the plot was needlessly overly dramatic and made Stan into a Designated Hero, culminating on a foreshadowed nonsensical song-and-dance number that, in the opinions of aggravated fans, only served to bash Stan.
A more meta example, but when the website's free full episodes switched to Hulu for streaming, fans were OUTRAGED.
Vocal anti-furries sometimes complain about their entire childhood being ruined forever after seeing fan art of old cartoon characters, whether sexual or not. Sometimes non-fanart furry art still gets this reaction, possibly stemming from the idea that anthropomorphic animals should be limited to kid shows only.
Similarly, Loonatics Unleashed got and continues to get this reaction from fans of the classic Looney Tunes. Kind of stupid once you consider that the characters on this show are technically different from the ones in the original shorts.
Several Batman fans were appalled after seeing the initial designs and more light-hearted tone of Batman: The Brave and the Bold. It's even lampshaded by Bat-Mite in the series finale.
Family Guy. About midway through the 7th season Brian began to become more preachy and more episodes began revolving around him and his political views. Other moments have gotten flak too (like the treatment of Meg), but Brian's soap box speeches have been the most consistently complained about and hated part of the show for the past few years.
After Augenblick Studios was unable to work on more Superjail! due to being busy with Ugly Americans, the animation production switched to Titmouse. The reactions from a number of fans could definitely be summed up with this.
There were some negative fan reviews of season 2 that also believed that the confirmation of Alice being Transsexual as being the ultimate middle finger from the creators to their guessing game about her gender, even though it had already been confirmed in interviews before season 2. Needless to say, although the show continues referencing Alice's bulge and plays with showing her transition more in flashbacks, Christy Karacas has admitted to sort of "regretting" revealing her backstory, possibly due to the backlash it received.
The Teen Titans animated series had received cries of this for the show having an anime-esque style, being more light-hearted and idealistic compared to the comics it was based off of, and for not taking place in the DCAU.
Terra is one very polarizing example. In the comics, she was clearly a mole who didn't actually consider herself a Titan and LOATHED them, and was a chain-smoking bitter sociopath. The cartoon version was toned down significantly, causing some long-term comic fans to cry foul at the writers for ruining a classic story arc. The fact that Terra didn't die and was instead petrified, and possibly returned? Only increased the fury.
Depending on how fans felt about Terra, the series finale "Things Change" also earned a rather negative reception.
Then comes the reboot, Teen Titans Go!, in which it has a more cutesy and chibi-ish artstyle, and the characters distilled to their base personalities, with all previous character development and insights gone. Other fans feel the emphasis on dark humor and a few episodes having a Cruel Twist Ending (reset button aside) have ruined it in the other direction.
Xiaolin Chronicles got this before it even aired, due to character designs being different (Raimundo isn't as attractive, and Dojo is yellow instead of green), characters (save Omi) being recast due to a different studio, and the Showdowns being depicted in CGI rather than 2D animation.
Adventure Time: In later episodes, and by that we mean every episode after the first season, many on the internet feel that the show is completely ruined any time episodes that depict shipping elements, contradict fanon, or where a character holds the Idiot Ball for plot movement air.
Many people on Tumblr have taken it upon themselves to criticise the show for things that were never seen as problems before. Namely the quality of the animation, the bright colors used for the characters (Lapis Lazuli gets the brunt of this) and the many Slice of Life "filler" episodes are often a target for critique.
On the subject of SpongeBob, many fans consider the series to have been Ruined Forever after creator Stephen Hillenburg stepped down as show runner, and finding the episodes following his departure to be worse than the old ones. Common complaints include Squidward becoming an even bigger Butt Monkey, Mr. Krabs becoming even worse thanPlankton, and SpongeBob and Patrick being dumber than before. The quality and merits of these episodes spark heated debates between fans, with many haters tending to hurl unflattering words at current showrunner Paul Tibbitt and writers Casey Alexander and Zeus Cervas.