Warhammer: The way Ogre mercenary Golgfag Maneater got his surname is an in-universe example. People started calling him Maneater after he settled a dispute with a human paymaster by eating him and walking away with his paychests. No big deal, except many people end up assuming he eats human meat and nothing else — which he doesn't — much to Golgfag's annoyance. Warhammer Ogres are Extreme Omnivores who'll eat literally anything when they're hungry (except gold, which is regarded as worthless due to lacking any nutritious value) and Golgfag is no exception, yet to this day he still has to grumpily explain to people who get the wrong idea that a) yes, he may eat a human if the mood strikes him, but b) no, he does not eat manflesh exclusively or have a particular taste for it.
Warhammer 40,000: It seems that some people seem to have taken the Clap Your Hands If You Believe element of Orky technology to mean that ALL of it runs simply because the Orks believe it does. No, a stick will not fire bullets if you convince an Ork that it can, goddamnit (though a pipe would, as far as the second edition rules are concerned, which was 5 editions ago and counting)! Being hit with a severe case of Depending on the Writer makes it even worse. (One source will say Ork tech works fine and the gestalt psychic field only makes it work better; meanwhile, another source will have an Ork fly a ship across a star system despite it lacking any fuel.)
The Imperial Guard will never escape their reputation as a Red Shirt Army who easily fall to any other force. Granted, that's mostly how they were depicted in the early editions, but later editions solidified their nature as a Badass Army fully capable of holding their own.
This can happen to writers too. Matt Ward was already unpopular but not necessarily loathed for being an Ultramarines fanboy who "hijacked the Space Marines codex to turn it into a 95% Ultramarines book" (the decision was more likely made from the opposite direction after it was too late for Ward to change anything). While a bad move, fans already accepted the overuse of Ultramarines as poster-boys and moved on. Then Matt made the Grey Knights codex, possibly the single most broken and overpowered army list yet conceived. Fan hatred of the codex was so far that members of the 4chan /tg/ board made lists of non-Grey Knights models that could be used to fashion a GK army without buying their specific models; all of this was based on the logic that Games Workshop would look at the sales of GK product to judge Ward's performance.
Sisters of Battle fans will likely never forgive that Grey Knights codex for having the GK massacre a convent and get off scot-free.
Another sore contention is Ward's complete overhaul of the Necron's lore. While received well by some players, many 'Cron fans believe that the army has gone from beingterrifyingEldritch Abomination worshipping killbots, to a comparatively lame mix between the Tomb Kings and a cheesy Doctor Who villain, and their star-eating gods have turned into Pokemon. Ironically, Ward is rather well-liked by Eldar players, because, while a horrid writer he may be, he's the only one who doesn't constantly submit the Eldar to humiliatingCurb Stomp Battles; in one of his stories, Craftworld Biel-tan successfully fights off two Imperial sector fleets and TEN Space Marine chapters by itself, although this isBiel-tan we're talking about, so...
Similar to Ward, Robin Cruddace will also never live down the "horrible atrocity" that was the 5th edition Tyranid Codex. Even the codex itself is only remembered for two things: introducing the nigh-useless Pyrovore and nerfing the beloved Carnifex to hell to make the newer Monstrous Creatures useful. This carried on even when the Tyranids got an update for 6th edition and the further nerfs it received had people burning Cruddace at the stake despite his name being nowhere on the book. note GW had, at this point, stopped naming individual authors on their books and instead just crediting the whole team as a result, as anything with Ward's name on it will instantly be decried as a defilement of fluff/broken-ass shit/Ultramarine Fanboyism/any combination thereof. The fact that the Tyranid codex got nerfed when they decided to do this pointed EVERYONE in Cruddace's direction.
Legends for the vanilla legends, but it's still known as a good set, despite them.
The Dark for Sorrow's Path, considered the worst card in Magic (at least, that isn't an outright joke).
Fallen Empires may have given us Hymn to Tourach, but it also gave us the ability to pay life to put a creature in play to put counters on that we could sacrifice to gain life equal to the number of counters. Guess which one got remembered more.
Kamigawa for the awkward "splice onto arcane" mechanic.
Despite Rule Zero technically protecting any given RPG from one bad rule ruining the bunch, splatbooks can have 200+ of great material and only be remembered for one bad rule or feature. It can get even worse if the feature isn't itself bad, but synergizes with another book's rules to create something unintended.
For example, Frostburn is a very well done feature book on cold weather terrain and characters in Dungeons & Dragons. At this point it will seemingly only be remembered for providing two minor feats that contribute to making the Locate City Bomb.
BattleTech has "Mad" Maximilian Liao. The man was canonically a legitimate Chessmaster in his younger years, may well have suffered from a medical condition explaining his decline if some of his descendants are any indication, and finally only really lost it at the very end of his career — but the fandom will always remember him for his "grab the wedding plates, they're military intelligence!" moment in the Warriortrilogy and being notionally played for a fool at every turn by Hanse Davion & Co. during the disastrous (for House Liao) Fourth Succession War.
The Lyran Commonwealth will always be remembered for their ''Social Generals'', who got their rank through money and connections than by experience. While the Lyrans have several formidable Mech Warriors and even the merger of the Federated Commonwealth gain them some reasonable commanders, the Lyrans are mostly known for simply sending Heavy and Assault mechs as their primary tactic.