WARNING: Unmarked Spoilers Below!
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The Human Child / The Player
- The fandom often portrays the Human Child as completely mute. In fact, this is impossible since they have phone conversations with the other characters (like Toriel) over the course of the game, occasionally speak in the form of player-chosen responses, and gives Asriel their true name, Frisk, of their own volition in the Golden Ending.
- 'Determination' is often cited with full confidence as the 'trait' for the red SOUL; however, in-game, the red SOUL's trait is never stated, and furthermore, it's mentioned several times over that all humans have Determination. Also, in-game, the only color Determination is assigned is yellow in Alphys' True Labs entries; every other time it's mentioned, it gets plain white text, and it isn't listed with the other traits during the Ball Game, although, curiously enough, 'Ball Game' is written in red text should you beat it with every other color.
Flowey
- Many think that due to being soulless, Flowey doesn't feel anything. In reality, however, a monster's SOUL is made out of love, compassion and empathy — And due to being soulless, Flowey can feel everything except those three things. He's actually shown to be scared, sad and even happy (although through sadistic means) throughout the game.
Sans
- The phrase "you dirty brother killer" is very often associated with the final battle of the Genocide route in which you fight Sans. While Sans does say this to you at one, and in the same location no less, he does not say it any time during said boss fight - it's actually an optional line of dialogue he can tell you if you in the final hallway in a neutral run in which you kill Papyrus, if you answer his Comes Great Responsibility question with "no". The confusion here is likely because every monster needs to be killed in order to get to Sans' boss fight, including Papyrus, and his death still plays a major thematic role regardless.
- Given how often it's depicted as such in fanworks, you'd be forgiven for thinking that whenever Sans's eye glows, it only glows blue. In fact, anytime his eye glows, it always flashes blue and yellow. There are zero instances in the game where it is only blue (or only yellow). Similarly, you'd be forgiven for thinking that the glow manifests as an emitted flame, when in-game it's depicted as a flashing pupil and the color never leaves the socket.
- Everyone knows Sans's Trademark Favorite Food is ketchup, right? While he does chug it in an optional scene, there's nothing that indicates it's his favorite food, and it's never brought up again. The reason this idea was perpetuated is probably the fact that you can find ketchup, mustard, and relish hidden in one of his sentry stations, but considering the fact that he later sells hot dogs to you, they're just as likely simply condiments for them.
- It's often believed that Sans has the ability to fully remember previous timelines, particularly ones where either his loved ones are killed or the happy ending is stolen away from him, leading to many a great Tear Jerker. Because of this, it comes as a surprise to some to find that he in fact does not have this ability in the same way your character or Flowey does; if anything, he only has the same vague, subconscious memories about them that every main character does and that's it. What sets Sans apart is the fact that he knows the previous timelines exist, and uses this information to make very specific (and uncannily accurate) guesses based on your body language and behavior as to what you've been doing in previous game sessions. Likely due to how specific (and frequently accurate) these guesses are, many people mistakenly got the impression that he actually has Ripple-Effect-Proof Memory like Flowey or the player, and it's difficult to discern the fans who genuinely believe it and those who just see it as a compelling "What if?" scenario. It's implied that he might have the ability to save physical objects from timelines that have been lost, judging by the contents of his secret room, but that's less definitive.
Papyrus
- Much like Sans and ketchup, spaghetti isn't Papyrus's Trademark Favorite Food. The anniversary Q & A
confirmed both that he makes it all the time because he thinks everyone else likes it, and that his actual favorite food is dinosaur-egg oatmeal. - Ask a fan what Papyrus's Berserk Button is and they'll likely tell you that he can't stand puns. In truth, though, he's one of the most pun-happy characters in the game; his phone calls show he has a pun ready for every occasion and he claims that Undyne's love of puns is one of the reasons why they're such good friends. He does get annoyed when Sans makes puns, but that annoyance is directed more towards his brother for shirking his duties to crack jokes than it is to the puns themselves— and even then Sans is quick to note that for all his complaining, Papyrus is still smiling.
Alphys
- It's often theorised that Alphys only wrote half of the True Lab's Apocalyptic Log, because the entries covering the discovery of Determination and the early experiments have proper capitalization while the later ones use all lowercase letters, implying that they were written by two different authors. However, Word of God heavily implies that Alphys was the one in the capitalized sections who coined the term "Determination"note , while the lowercase sections mention details like her building a new body for Mettaton, making it far more likely that all of the entries were written by her. As for why the grammar changes midway through, the much simpler answer is that it's a sign of Alphys' deteriorating mental state; the entries first slip into lowercase when it seems like all of her experiments failed, briefly return to normal when the test subjects regain consiousness and appear to be fine, then return to lowercase for good once they start melting together.
Asgore
- A search of the game's music and its many, many covers might give you the impression that Asgore's battle theme is titled "Bergentrückung". It's not; "Bergentrückung" is the name of the short intro section that plays before the battle, during the cutscene where he destroys the MERCY button. His actual battle theme is just called "ASGORE".
Muffet
- Certain fan productions, including Undertale the Musical, depict Flowey as bribing Muffet into attacking you midway through Hotland. It's never made specific who paid her in-game, but it's heavily implied to be Mettaton; she mentions that the person who warned her about the player character was able to offer a lot of money, has a sweet smile, and is capable of shapeshifting — while the last traits do apply to Flowey (and is likely the source of this belief), the only known character that hits all three is Mettaton.
- The reason why Muffet attacks you is because she holds a grudge against humans for disliking spiders, right? Not really- it's because she was paid to and was told the player character specifically hates spiders and actively antagonizes them. Once she realizes she was wrong about you, she calls off the battle and lets you pass without so much as a spiteful remark. This is why using an item from the Spider Bake Sale immediately ends the fight - she realizes you can't hate spiders if you were willing to spend money to improve their lives. If you somehow manage to buy the Spider Donut in Hotlands for 9999G, she won't even initiate the fight to begin with and lets you go on your merry way, thanking you for your support.
The Fallen Human
- It's very common for fans to assert that the Fallen Human/Chara deliberately jumped into the underground in a failed suicide attempt, especially as an arguing point for viewing them sympathetically. While Asriel does state that they climbed Mt. Ebott for, quote, 'not a happy reason', we clearly see in the intro that Chara accidentally fell into the hole leading underground by tripping on a root (and, no, Flowey's not responsible because he didn't exist yet).
Dr. W.D. Gaster
Since what little information the game has on Dr. Gaster is hidden behind rare Random Events, most fans only learned of him through other fans. Because of this, popular fanon about him is often mistakenly assumed to be canon, despite not being confirmed in any official capacity:
- There's no definitive proof that he was retroactively erased from history, despite it being treated as a cornerstone of the mythos surrounding him. What we are told is that he "shattered across time and space" after one of his experiments went awry, but no mention is ever made of him being forgotten as a result; on the contrary, two of his followers claim that Asgore took a long time to find a new Royal Scientist who could live up to Gaster's legacy, implying that he was still remembered after he disappeared. This idea seems to have originated from Goner Kid's comments about a world where everything's the same but you don't exist; they don't mention Gaster at any point, but as they resembled one of his followers and could be encountered in the same way as them, fans assumed they must have been talking about him.
- Another popular idea related to Gaster's erasure is that Sans is the only one who remembers him, and has a drawing of him hidden away labelled "don't forget". This is partially true, in that as of the v1.001 update you can find a drawing labelled "don't forget" in Sans's workshop. However, this drawing is never shown to the player and is only ever described as "a poorly drawn picture of three smiling people", with no indication that one of them is supposed to be Gaster. If anything, considering all of the details surrounding it that serve as Production Foreshadowing for Deltarunenote , it seems more likely that the drawing is meant to depict the three heroes from that game.
- It's widely believed that Gaster's fate was the result of him falling into the CORE. While there is strong evidence supporting this, with one of his followers describing how "his life was cut short" when he "fell into his creation" right after stating he built the CORE, implying that was the "creation" in question, the wording is just vague enough that it isn't certain. It could have been the CORE he fell into, but it just as easily could have been whatever he was documenting in Entry 17, or one of many other "creations" he likely made during his tenure as Royal Scientist.
- Since the True Lab is so heavily associated with Gaster, many fans assume that he played an active role in the Determination experiments that took place within its walls and the subsequent creation of the Amalgamates. However, reading through Alphys's lab entries reveals that she was the one who first discovered Determination after she was appointed as Royal Scientist, by which point Gaster was already long gone.
- The idea that Gaster is stranded in a Void Between the Worlds is such a ubiquitous part of so many fan works that it's often believed to have some basis in canon. In reality, the existence of such a "Void" is never once mentioned or even alluded to in-game.
Other
- Many people believe that buttercups and golden flowers are the same species. They can't actually be because, if they were, Chara's fake request to see the golden flowers would have been easily fulfilled since buttercups already grew in the Underground as evidenced by them and Asriel putting them into a pie once. Furthermore, Alphys's True Lab entries state that the golden flower that would become Flowey was the first one to grow in the Underground and that it did so not long before Toriel left.
- Due to the fact that it's commonly referred to as the "No Mercy" route, many players believe that sparing a single monster will end the Genocide route immediately. Sparing a Random Encounter (with a few exceptions) will not end the route, though it also won't help you progress. One strategy to make progress faster actually involves sparing the hard-to-kill Stone Wall-type monsters so you can move on to fighting easier enemies.
- It's very common for critics of the game to accuse it of Blamed for Being Railroaded for the Genocide Route, ignoring the fact that the player can terminate the Genocide Route at any time (with much of it being a slog that the player needs extreme determination to overcome), and much of the You Bastard! moments from that route being entirely in-universe from the monsters having surprisingly realistic reactions to the player's rampage (as opposed to the game itself accusing the player of being a psychopath, as they often imply). Sans and Flowey do both accuse the player of only massacring everyone because they were bored and wanting to see every possible route (with Flowey also even calling out people who watch gameplay footage of the Genocide Route but are too afraid to play it themselves), but both of these are more just incredibly dark Lampshade Hanging than actual judges of the player's character.
- A surprisingly common belief in gaming circles is that the game was made with RPG Maker. It was actually made entirely in Game Maker Studio. While it is true that RPG Maker has been purpose-built for creating the sort of SNES-era JRPG Undertale has clearly been Inspired by…, it doesn't handle real-time action well: though there are some Action RPG games made with it like Avarice (or even a straight-up Shoot 'Em Up like SkyE), its turn-based battle interface is quite rigid, and interspersing it with the real-time dodging segments that became Undertale's calling card would have been too complicated to bother with.
