%% Note: The route for the worst ending, aka the "No Mercy" route, aka the "Genocide" Route, does NOT have an official name. Therefore, call it whatever you want. To avoid edit wars, please don't make edits that do nothing but change one term to the other. The wiki isn't the official source of information for the game.

* BeamMeUpScotty:
** The game's save points fill you with "determination" in regular lowercase letters. Many people mimicking the style of the save points use "DETERMINATION" in all caps, which doesn't show up until late in the game.
** Many people quote Sans as saying "You're gonna have a bad time" late in the game. He actually says this much earlier in the game as a warning to stop doing the No Mercy run. His specific quote later in the game is "Do you wanna have a bad time?"
** The phrase "you dirty brother killer" is often associated with Sans during the OmnicidalManiac Genocide route. Thing is, Sans never says that in the Genocide route; he only says this at the end of the ''Neutral'' path if you killed Papyrus and then say "no" to the question he asks.
** The infamous "get dunked on" GameOver screen is associated with whenever the Genocide final boss kills you. However, this only applies to his ISurrenderSuckers tactic whenever you decide to spare him.
** In the True Pacifist ending, [[spoiler:Asriel never tells Frisk not to trust himself-as-Flowey if Frisk ever runs into him again. He just says to think of Flowey as a different person from Asriel. He does say, when describing the outside world, that "there are a lot of Floweys out there", but he's clearly using his past behavior as a metaphor there.]]
** While it's not quoted particularly often, [[spoiler:Flowey's line if you choose to kill him after his boss fight]] is occasionally given in all-caps with a period at the end: [[spoiler:"I KNEW YOU HAD IT IN YOU."]] In-game, it's in mixed-case with an exclamation point: [[spoiler:"I knew you had it in you!"]]
* BootstrappedLeitmotif: Most animated fanworks and fangames involving [[spoiler:Chara/The Fallen Child]] have "Megalo Strike Back" or a remix of it as their theme. While the song doesn't originate from ''Undertale'' itself, it was one of Toby Fox's earlier compositions from the ''VideoGame/{{EarthBound|1994}}'' fan album ''I Miss You - Earthbound 2012''. The fandom decided it was fair game since Toby himself used another of his songs from the album ("Fallen Down") in the game. "Star", a CutSong meant to play after dodging one of Madjick's attacks for long enough, also gained popularity as [[spoiler:Chara's]] theme due to several musicians bringing that idea to prominence in the years after the game's release.
* BreakthroughHit: For Creator/TobyFox. Granted, Toby is more of a musician than a game developer by trade, as seen by his contributions to ''WebComic/{{Homestuck}}'' and an ''VideoGame/{{EarthBound|1994}}'' tribute album.
* ColbertBump:
** Once the ''Undertale'' fandom decided to push the game to victory at a Website/GameFAQs [[http://www.gamefaqs.com/features/bge20 "Best. Game. Ever." contest]], [[http://www.gamefaqs.com/poll/6182-did-the-best-game-ever-contest-convince-you-to-try-undertale a few of the website users went on to pick it up]].
** Closer to the game's launch, several [[LetsPlay Let's Players]] decided to play through the game, most notably ''WebVideo/SteamTrain'' and ''WebVideo/TwoBestFriendsPlay''. Both of them gave the fandom a huge boost of popularity with their several thousand strong fanbases who were equally captivated by the game. Joel from ''WebVideo/{{Vinesauce}}'' was also personally contacted by Creator/TobyFox, asking him to play the game (not only does he have a huge fanbase, but skeletons and puns are AuthorAppeal for him).
* CreatorBacklash: Minor example. Creator/TobyFox admitted in [[https://www.twitter.com/tobyfox/status/826070605376151554 a tweet]] that he was annoyed by the player character's slow movement speed, but thought adding a run button retroactively would mess up the timing of many scenes.
* CreatorsPest: If the jokes the game makes about it are meant to be taken at face value, then it appears that Creator/TemmieChang's least favourite monster is [[TestosteronePoisoning Aaron]]. The True Pacifist credits claim she designed him "reluctantly", and making Aaron appear by flexing during a fight with [[AuthorAvatar a Temmie]] will cause her to run away in disgust.
* CrowdfunderCameo: Glyde, So Sorry, and Muffet were designed by Kickstarter backers, and so were a few [=NPCs=].
* CutSong: Quite a few.
** A song called "Star", using instruments from the original ''VideoGame/StarFox1''. [[WordOfGod According to Toby]], it was originally associated with Madjick's special attack.
** There's a 'song' called "Smile" that plays during the "Entry Number 17" event that can only be found by editing your save file or through debug mode. It's actually Muffet's laugh slowed down 1000% and pitched down.
** There's a regal sounding variation of "Determination" (the song that plays on the Game Over screen) called "King Description" in the game files.
** "Grandpa Semi" sounds remarkably like a very simple version of the beginning of "Metal Crusher". As of v1.001 the file has been renamed "grandpatemi" and plays if you refuse a Temmie some Temmie Flakes during a battle.
** "Dance of Dog" and "Sigh of Dog" can be found by messing with your save file to put yourself in rooms that don't exist, giving you an error screen with the Annoying Dog either dancing or sleeping respectively.
** Unused parts of "Finale" and "Your Best Nightmare" exist in the code, either to shorten or lengthen the songs.
** A variation on the Ruins theme, "Ruins Piano", which is a slower version of "Ruins" and has a slight variation on the melody and sounds a bit like "An Ending".
** "mus_piano" actually exists on the Soundtrack as "Good Night" and was used in the demo but goes unused in the full game.
** Then there's the 'Sound Test' songs, "Meat Factory", "Trouble Dingle", "Happy Town", and "Gaster's Theme", which ''technically'' exist in-game but only through the ludicrously hard to trigger Sound Test Room in Snowdin Town.
** There's also unused variations of songs that were revealed on the first anniversary of the game; namely a variation of Alphys's theme that was cut for sounding too much like [[VideoGame/EarthBound1994 a lab song from another game]], an early version of "Spear of Justice" that was cut for not sounding like "being heroically punched in the face", "Dog Hole" (no description for this one), "Dogtroid" was supposedly meant to play if one did a speedrun of the game and was rewarded with a [[{{Franchise/Metroid}} picture of the Annoying Dog in a bikini]], an unused version of "Undertale" that was more melancholic and "less effective", and "Patient" was reworked into "Another Medium".
* {{Defictionalization}}: Because of the vast amount of healing items in the game, nearly every food in the game has at least one fan recipe. Spider cider donuts (but not made with [[MadeFromRealGirlScouts actual spiders]]), Toriel's cinnamon-butterscotch pie, Papyrus-inspired spaghetti, spicy Grillby-inspired fries, Sea Tea, etc. At least one Tumblr is trying to actually compile a fan-made ''Undertale'' cookbook!
* DescendedCreator: After he watched a particularly hilarious freakout by Hololive Vtuber Houshou Marine to Flowey speaking in the Genocide route, [[https://mobile.twitter.com/houshoumarine/status/1364497882816991239 Toby revealed to her in a DM that for the Japanese version of the game he voiced Flowey himself]].
* DummiedOut: Quite a bit of [[https://tcrf.net/Undertale cut content]] can be found by hacking into the game:
** There's battle text and sprites for a [[https://www.youtube.com/watch?v=iec10EIL9QY female dog guard named Doge]] and [[https://www.youtube.com/watch?v=lCXYb7X-vLM two female Royal Guard members]] based on a cat and a bug.
** The mystery of Dr. W.D. Gaster is centered around a bunch of [=NPCs=], music, and rooms that were initially only found through hacking, but with the 1.001 update, there's now a small chance that most of them can be encountered normally.
** The protagonist has a set of sprites with HiddenEyes and a green-yellow striped shirt. This might have been intended as [[TheMirrorShowsYourTrueSelf their reflection]] [[spoiler:after completing a murder run and selling their SOUL]], but it only actually appears with a DebugMode option on, and even then only in one room in Waterfall. Elements of this design would later be incorporated into Kris' Light World appearance in ''VideoGame/{{Deltarune}}''.
** {{Parodied|Trope}} with a song on the Official Soundtrack titled "Song That Might Play When You Fight Sans". It doesn't show up in the finished game's files, thus it does not play when you fight Sans.
** In the demo only, there's a couple instances of text and file names referring to a "Grandpa Semi". The inclusion of semi-serif fonts in the demo suggests that another skeleton character was planned at some point. The "grandpasemi" name was eventually renamed to "grandpatemi" and repurposed for the Temmie enemy, if you annoy one by denying her Tem Flakes.
** There's a unused regal-sounding remix of the GameOver theme within the game files
** A short, upbeat tune can be found within the game files. It only plays during a placeholder screen involving the Annoying Dog that only appears if something goes wrong. A different song will play for a similar purpose, being a lullaby and showing the Annoying Dog sleeping on the screen.
** There are a few unused enemy formations, including [[TheFriendNobodyLikes Jerry]] by himself; presumably cut because it would be an annoying and pointless encounter. Though, given that it's Jerry, that might also be why it was considered in the first place.
** The "dirty hacker" ending is an exploited example; if you play the game without hacking and see it anyway, something has gone horribly wrong.
** Mettaton EX has responses for if you type "sexy", "foxy", or "tantalizing" in his essay question, but since the X and Z keys do nothing here for obscure technical reasons, you can't ever see them.
** An accidental example: At the beginning of the game, a random "fun" value is written into your ini file. Throughout the game, certain EasterEgg events can appear depending on the "Fun" value written in your ini. Unfortunately, most of them were case-sensitive; since they were looking for a particular "'''F'''un" value, they would completely ignore your "'''f'''un" value, unless you manually went in and edited it. Fixed in 1.001.
** "abc_123_a.ogg" is an exploited example; dataminers would see it at the top of the music listing and find a message telling them to "have some respect and don't spoil the game". "abc_1111_0.png" is a similar warning in the sprite sheets, but is less easily found due to the way Game Maker works. Both were replaced with unrelated nonsense in 1.001, as the game had been out for a while by then and Toby was only really worried about spoilers getting out before people could experience things themselves.
** Toriel has a sprite that is similar to her defeated battle sprite and the file name has suicide in it, which implies that [[WhatCouldHaveBeen Toriel might've killed herself after the battle or sometime later]].
** There are seven variations of the main menu theme, but only six are used in-game. The last of them — essentially the fifth menu theme with an added glockenspiel — is found between the used "Alphys" and "Pacifist Ending" versions of the song, the fifth and seventh variants, respectively; this implies it was meant to be used for Alphys, and another friendly character (most likely Mettaton, who can't be befriended in the final game) would have used the fifth version instead. It goes unused simply because there's nowhere for it ''to'' be used.
** [[https://www.reddit.com/r/Underminers/comments/3tj4e7/unused_text_i_havent_seen_mentioned/ Two pieces of unused text have been found.]] One involves Mettaton sending you a "Mortal Enemy Request" if you deny Napstablook's friend request in Hotland (in the final game, you get the same response as if you'd accepted it); the other involves a history book entry defining what is meant when a monster "Falls Down" (while the concept is only ever mentioned in passing in the final game, the definition — comatose and near death from old age — [[IKnewIt is in line with what fans suspected]]).
** A special song called [[https://youtu.be/PkSEgbY_ey4 "Star"]] was meant to play if you endured Madjick's chaser orb attack long enough for the attack to turn into hearts that would restore your health. It was cut due to resulting in glitches with the normal CORE background music.
** In the game's official newsletter, Toby admitted that he had originally planned an alternate version of the scene where Sans judges the player for speedrunners; basically, if the player had reached the Last Corridor in a certain, unspecified amount of time since starting the game, an alternate version of the scene would play where Sans would spend [[TrollingCreator upwards of almost two full minutes doing nothing but eating an ice cream cone]] before immediately letting the player go while lamenting that they [[LeaningOnTheFourthWall clearly have something important to do and he wouldn't want to keep them waiting]]. This scrapped version of the scene can be found [[https://youtu.be/BsuH5UPaCYw?si=ksmNF-e8TnmxZ9jv here]] along with more info; interestingly enough, the sprites of Sans eating ice cream can still be found in the full release's files.
* DevelopmentGag: According to a quote from the official artbook, the whole gag with Lesser Dog's LongNeck stemmed from an in-joke between Toby and Temmie when they were first working on its design:
--> '''Toby:''' I told Temmie to draw a smaller version of Greater Dog as a counterpart, so she did... but her sketch had this like, weirdly long and puffy neck. So every day after I told her that I had some important concept art to look at. And then every time it would just be her drawing of the dog, but with its neck badly extended in MS Paint. Until it just reached this point and its neck was really long. Anyway that’s where the idea for Lesser Dog came from. [[{{Troll}} Just annoying Temmie]].
* FanworkBan: Toby Fox had to enact a mild one regarding physical merchandise in response to the game's unexpected popularity, similar to the moratorium on ''Webcomic/{{Homestuck}}'' fan merchandise. Read it [[http://undertale.tumblr.com/post/139726155020/changing-policy-on-fan-merch here]]. In a nutshell, mass-produced goods and anything that would be directly competing with official merchandise, such as Toriel plushies or clothing articles of any kind, are disallowed, but everything else (including one-off commissions of graphic art) is fair game.
* GodNeverSaidThat:
** Creator/TobyFox denied having based Papyrus' personality on a [=YouTuber=] known as Octopimp after a wiki erroneously reported that he had.
---> "[[https://web.archive.org/web/20151116035337if_/https://twitter.com/FwugRadiation/status/661228770540523520 uhhhhhhhh........ no LOL? deconfirmed?]]"
** Toby actually had to [[https://twitter.com/tobyfox/status/708639129924444160 clarify]] that a certain joke post was, in fact, a joke.
** As noted under UrbanLegendOfZelda, despite widespread [[WordOfDante fandom assumption]], the 'spr_mysteryman_0' sprite has not been confirmed to be W.D. Gaster.
** An old post on Tumblr asserted that Toby privately objects to the "Genocide" name and wishes people would call it something else. While Toby hasn't ''de''confirmed this claim (seemingly preferring to [[ShrugOfGod stay out of that debate entirely]]), the post stopped going around when people noticed that OP didn't quote the alleged email and no other source for this claim could be found.
* HalfRememberedHomage: Toby Fox has said that ''VideoGame/MoonRemixRPGAdventure'' was a major inspiration for this game. He has also said he never played the game before developing ''Undertale''. His exposure was limited to hearing about how you progress in the game through helping others instead of killing.
* HeAlsoDid: Before making ''Undertale'', Toby Fox was known for composing much of the music used in ''Webcomic/{{Homestuck}}''. Everdraed, too, is mostly recognized for making extremely complex photo-manipulated gifs (often with embedded sound) on [=SomethingAwful=], [=YouTube=], and Twitter, so his work on the Neutral final boss comes as little surprise.
* HeyItsThatSound:
** The game primarily uses Platform/Super Nintendo|EntertainmentSystem soundfonts for a majority of its soundtrack. Players who listen closely can recognize instruments from the various games used. Obviously, ''VideoGame/{{EarthBound|1994}}'''s soundfont is used most often, but other games are used as well. For example, "Reunited" uses instruments from ''VideoGame/ChronoTrigger'', "Oh! One True Love" uses ''VideoGame/FinalFantasyVI's'' instruments, and "Song That Might Play When You Fight Sans" uses ''VideoGame/MegaManX3's'' instruments.
** Flowey's laughter is a stock laughter effect, albeit manipulated to sound like it does in-game. The original sound effect was also used in ''{{VideoGame/Tomba}}'', [[https://www.youtube.com/watch?v=2aBa-iQSkxc where strange red flowers would laugh and gain sinister facial expressions after the titular character ate a mushroom]]; in ''[[VideoGame/DrBrain Dr. Brain: Action Reaction]]'', [[https://www.youtube.com/watch?v=9J8X-6ycDoE#t=51m20s in the playable credits/circus level]]; and in [[VideoGame/TheJackboxPartyPack Jackbox]]'s ''Fakin It'' [[https://jackboxgames.bandcamp.com/track/fakin-it-credits-song during the credits]].
** Flowey's "That's a wonderful idea!" is [[https://www.youtube.com/watch?v=GV1VuhiW9p4annotation_id=annotation_4133518649 a distorted clip]] of [[UsefulNotes/McDonalds Ronald McDonald]] saying the same thing in a [=McDonald's=] commercial from the 1960s. [[spoiler:Photoshop Flowey's voice clips are edited from [[https://youtu.be/TCyNIKA-uhs?t=133 the same commercial.]]]] Averted in the Japanese version, where Flowey is voiced by Toby himself.
** Sans' dialogue sound is edited from a voice clip of [[WesternAnimation/SpongeBobSquarePants Patrick Star]] saying [[Recap/SpongeBobSquarePantsS2E2SomethingSmellsBossyBoots "Maybe it's the way you're dressed"]], [[https://soundcloud.com/undertale-recreated/undertale-bonus-track-sans-voice-recreation cropped to the first "a" sound]] and looped repeatedly.
** Fallen Down [[https://sleepytimejesse.bandcamp.com/track/fallen-down previously appeared]] on an ''VideoGame/{{EarthBound|1994}}'' fan-mix album.
* InspirationForTheWork: Toby Fox has been fairly open about the role-playing games that inspired ''Undertale'', beyond obvious ones like the ''MOTHER'' series:
** ''VideoGame/MoonRemixRPGAdventure'' was [[https://twitter.com/tobyfox/status/931655092264108032?t=JPby6ZY-JWzs3srFF5SMRg&s=19 a major inspiration]] for the "anti-RPG" aspects of ''Undertale'', with both games using similar concepts such as [[WhatMeasureIsAMook friendly monsters being killed by a archetypal RPG hero]] and "Love" as a replacement for CharacterLevel (though the way it's portrayed in each is completely opposite). Notably, Toby hadn't played the game himself during the development of ''Undertale'', as it was not officially translated into English until a few years later.
** While already known for being the source of Toby composing "Megalovania", ''VideoGame/LiveALive'' inspired ''Undertale'' in other ways as well (some covered in [[https://www.square-enix-games.com/en_US/news/live-a-live-yoko-shimomura-toby-fox-pt-1?utm_source=sqex-social&utm_campaign=LIVEALIVE&utm_content=Toby+Fox+Interview+Pt1 this interview]]), with Toby directly saying that its Twilight of Edo Japan chapter influenced the pacifism-versus-violence gameplay of ''Undertale''. He's also said that aspects of the story, particularly [[spoiler:Oersted's ProtagonistJourneyToVillain twist]], influenced the character writing of ''Undertale'' and subsequently ''VideoGame/{{Deltarune}}''.
* MemeAcknowledgment:
** After Sans narrowly won in the finals of a September 2022 "[[Memes/{{Tumblr}} Tumblr Sexyman]]"[[note]]in short: characters who are weird- or even only average-looking, who nevertheless explode in popularity on Tumblr and get disproportionately big diehard fanbases who find them [[PerverseSexualLust more attractive than they really are meant to]][[/note]] Twitter poll against [[Webcomic/MobPsycho100 Reigen]], fans celebrated the victory the way they know best - [[https://www.tumblr.com/tagged/sans%20sweep?sort=top by making memes about it]]. To the surprise of all, even Toby Fox himself joined in on the fun by writing a [[https://twitter.com/tobyfox/status/1567739718762364929?t=NwGy0zF2_uQ9PaNW5JX0tw&s=19 short fanfic]] where Sans beats Reigen by one vote in a "Tumblr Sexyman" contest. He would also create [[https://twitter.com/tobyfox/status/1744872613543805314 art of them over a year later]].
** When [[WebVideo/GameTheory MatPat]] announced that he was stepping down from ''The Game Theorists'' in 2024, Toby Fox responded by wishing him well and posting art of Sans and Papyrus dressed in Ness outfits, referencing the infamous "Sans is Ness" theory from [[https://twitter.com/tobyfox/status/1745048357846859798 nearly 8 years prior]].
* OutOfHolidayEpisode: An alarm clock companion app was planned as a vehicle for various humorous monologues from the characters. Development of this app was halted to focus on ''VideoGame/{{Deltarune}}'', so in September 2020, Creator/TobyFox took the ChristmasEpisode portion of the dialogue, polished it up, and [[https://undertale.com/alarmclock/ published the dialogue by itself]]. [[spoiler:At the end of Asgore's page, Flowey pops in to incredulously wonder how [[ChristmasInJuly nobody noticed the date]].]]
* PermanentPlaceholder: According to the official art book, Napstablook was created as [[https://tcrf.net/Prerelease:Undertale/Concept_Art#Napstablook a placeholder character]] to help conceptualize the game's battle layout. They ended up being so popular with playtesters that Toby turned them into a proper character.
* RecycledSoundtrack:
** "MEGALOVANIA", a song previously used in the ''VideoGame/TheHalloweenHack'' and ''Homestuck'', returns as a remix to play during the Genocide final boss.
** A part of Metal Crusher was originally composed as a [[http://tobyfox.net/msuics/noisemaster_lessecho.mp3 fan theme]] for [[Webcomic/CucumberQuest Noisemaster]].
** [[http://existentialgamer.com/interview-toby-fox-of-undertale In an interview]] Toby mentions that "Another Medium", "Heartache", "Fallen Down" (as mentioned above) and "Nyeh Heh Heh"/"Bonetrousle" were all songs he made for other projects that got recycled into ''Undertale''. In particular, "Heartache" and "Nyeh Heh Heh"/"Bonetrousle" were originally composed for ''VideoGame/{{Deltarune}}'',[[note]]the former was [[spoiler:Jevil's original battle theme]] before being replaced by [[spoiler:"THE WORLD REVOLVING"]], and the latter two are based on the original regular battle theme that "Rude Buster" replaced[[/note]] which started development a year before ''Undertale'' did only to be put on hold until after ''Undertale'' released.
* ReferencedBy: Enough works to [[ReferencedBy/{{Undertale}} get its own page]].
* RuleThirtyFourCreatorReactions:
** Possibly more preemptive than a reaction, but [[http://undertale.tumblr.com/post/119606517315/undertale-release-not-extremely-far this blog post]] seems to apply. Rather than saying not to make it entirely, it simply tells Tumblr users not to put tags on it that would cause it to show up along with the official developer posts and "clean" fanart when someone searches for "#undertale", and instead use "#underta'''il'''" (because Toby Fox was aware that there would be kids playing the game and didn't want them to come across porn during an innocent search for their favorite characters).
** Character designer Creator/TemmieChang is more complicated about the subject. She's [[https://twitter.com/tuyoki/status/675513730764001280 stated]] on Twitter that she dislikes pornography that features Temmies (as they're her AuthorAvatar) and will likely intervene with porn blogs focusing on them; she's additionally [[https://twitter.com/tuyoki/status/675516692974448640 asked]] that all Temmie porn be tagged "Temmoi."
** In the game proper, there are some references to the Rule 34 art as well. For one, one late-game conversation involving Toriel ends up subtly commenting on the type of art fans make of her (in spite of her age) while still staying within the fourth wall. The other is when Sans makes a small reference to the game's UsefulNotes/FurryFandom if you persist in buying "hot dogs" from him. In both occasions, it's just a light jab at the fandom with no real contempt towards what they enjoy.
---> '''Sans:''' ... you really like [[DoubleEntendre hot animals]], don't you? hey, i'm not judging. i'd be out of a job without folks like you.
* SleeperHit: Not even Toby himself expected the immense success ''Undertale'' would get when he released it.
* ThrowItIn: [[spoiler:The Fallen Human's shirt in the opening cutscene {{foreshadowing}} the fact that they're not the playable character]] came about thanks to a mistake Temmie Chang made while storyboarding the sequence, accidentally drawing [[spoiler:their shirt with the wrong number of stripes]]; Toby Fox ended up liking the error and worked it into the game's narrative.
* UrbanLegendOfZelda: Attracted so many that [[UrbanLegendOfZelda/{{Undertale}} it has its own page]].
* WhatCouldHaveBeen:
** The demo's instruction manual can change in response to your actions in the game. This was removed when the game switched to the Studio version of Game Maker, which doesn't have the functionality needed to write images to your hard drive.
** The demo featured a mechanic where you'd get partial EXP from enemies you damaged and then Spared. This partially nullified the main gimmick of a Pacifist run and promoted grinding, so it's not too surprising to see that it got cut. [[spoiler:Well, that and the fact that EXP is actually Execution Points, so gaining it from not executing enemies wouldn't make sense. The demo manual does carefully refer to EXP as "Ex. Points", which means that either Toby hadn't yet noticed the FridgeLogic or he had a different "Ex" word in mind, though nobody seems to have come up with one that would make more sense.]]
** The final boss of the No Mercy route was going to be ''harder'', with one of the InterfaceScrew attacks (attacking the player's menu cursor) being able to kill. Thankfully, the attack was limited to reducing HP to 1.
** On the ''Undertale'' Kickstarter page, the DatingSim segments are mentioned, alongside a statement that "Seriously, you can literally have a robot husband." This seems to be referring to Mettaton (who hadn't been revealed yet), but there's no option to date him in the final game. It may have been cut for plot reasons, because [[spoiler:by the time you make peace with Mettaton, he's out of commission until after the True Pacifist final boss]].
** Within the game files, there's an unused rough draft of a "Spell" option for the battle menu, indicating that you might have been able to learn magic spells of some kind. In addition, a screenshot showing a pre-alpha build of the game ''with'' the aforementioned icon reveals "Act" and "Mercy" were originally called "Talk" and "Spare", respectively. The "Spell" option would appear in ''VideoGame/{{Deltarune}}'' for Ralsei and Susie in place of "Act" for Kris.
** [[https://twitter.com/FwugRadiation/status/686928671429541891 According to Toby]], Mettaton originally literally saved your essays about him for future reference; specifically, he'd save them to your hard drive. It was cut for being buggy in some way.
** [[https://web.archive.org/web/20160223131557/https://mobile.twitter.com/tobyfox/status/686934823563345920 A comment in the game's code that Toby posted]] suggests that Muffet was going to have croissants that attacked vertically as well as horizontally. Given Toby's [[PrecisionFStrike strong language]] in said comment, they were most likely removed for being too hard to dodge.
** It was originally planned to have a second pair of royal guards, RG 03 and 04, who would replace RG 01 and 02 depending on the day of the week. Much like the Pacifist solution for 1 and 2 was to get 1 to confess his love to 2, the Pacifist solution to 3 and 4 would be convincing them to forgive each other for an argument and become friends again.
** Toriel was probably planned to commit suicide according to a DummiedOut sprite of her that has suicide in the file name. The idea was likely scrapped due to the concept being too dark too early in the game, but it was pushed onto [[spoiler:Asgore, if the player attempts to spare him after having killed Flowey previously.]]
** The main menu theme has [[EvolvingMusic six variations]] which play depending on where you are in the game and how many characters you've befriended. Another variation exists in the game files between the fifth and sixth used songs, suggesting there was meant to be a dating sequence with a seventh character. (This was most likely Mettaton given several comments during the game's development process, but there's no opportunity for the player to make friends with him in-game since he's out of commision after his boss fight.)
** The official Undertale artbook reveals a lot of character designs that were planned and either changed or scrapped:
*** The playable human character originally was going to have a black outline on their sprites, but this was changed to match the color of their hair since most of the game was in a dark underground cavern and a black outline would blend in too easily.
*** Toby was originally going to have the player kill Toriel in order to proceed in the story. As he wrote Toriel's dialogue, he felt like killing her was just wrong and from there, he rolled with the idea of making ''Undertale'' a game where you can progress without having to kill anyone. Toriel's first design was something vaguely human before Toby changed it to a goat design. Toriel was also designed without horns, but Toby noted that she looked too similar to the mimigas from ''VideoGame/CaveStory''. He was partially inspired by the mimiga design and loves ''Cave Story''.
*** Papyrus originally was going to be quite mean-spirited, wear a fedora, and would have little to no redeeming qualities about him. Sans was going to wear some sort of visor hat, possibly having him run some sort of casino, and he was going to tell a lot more bone related puns.
*** The female bunny shopkeeper in Snowdin had an old, grandmother-looking design, and also wore a dress and a bonnet.
*** Asgore originally had no facial hair, and his hair in general was supposed to be black or brown, but was changed to blonde in order to make him stand out more.
*** Toriel's overworld sprite went through several changes. Her outfit was originally some sort of poofy dress with a scarf draped around her neck. The dress was made less and less poofy until it was changed into the robe Toriel wears in the final version, and the scarf was removed entirely.
*** Alphys was going to be male, but Toby didn't like how the design came out, so he stuck on some eyelashes and made Alphys female instead.
** [[http://imgur.com/a/ZWdoO Toby planned]] to have it so that if you managed to beat Asgore in one hit that Flowey would appear, dumbfounded, announce you won, then leave while silly music played.
** The game also [[http://imgur.com/a/xwaOS originally]] had a [[http://imgur.com/a/qpiNJ karma system]], and was [[http://imgur.com/a/5i678 generally]] ''very'' [[http://imgur.com/a/FTCga different]] from the final product.
** The above also mentions a "phasing" mechanic, and the different bullet colors would have revolved around how they react to phasing. White bullets could have been phased through, orange bullets would only hurt you when phased, blue bullets presumably would have disabled phasing, and the completely unused yellow bullet would hit you regardless of if you phased or not. The blue SOUL was also originally green, and would have replaced your phasing ability with jumping, although no mention is made of if gravity was different between these modes.
** [[http://www.nintendolife.com/news/2016/02/undertale_creator_open_to_nintendo_port_but_wouldnt_be_able_to_code_it_himself Toby spoke to]] Nintendo about the possibility of creating a port of ''Undertale'' for the Platform/Nintendo3DS or Platform/WiiU, but said it would be difficult as neither console supported Game Maker, and the Wii U itself was already on the way out the door by the time the request was made. The game was eventually ported to the Platform/NintendoSwitch, with a new boss fight which made use of the Joy-Cons.
** Within the game's data are [[https://twitter.com/tobyfoxtrivia/status/1614654894488567809 remnants]] for an unused mechanic where completing a Genocide playthrough would cause the game to delete itself. The idea was abandoned due to Toby being unable to find a way to make it work.
** According to the Summer 2023 Undertale Newsletter, there was originally going to be an alternate "judgment" that would occur if you made it to the judgment hall quickly enough, seemingly intended to Troll speedrunners. Sans would note that you reached him so quickly, he wasn't even able to finish his ice cream, and the player would have to [[OverlyLongGag watch him eat his ice cream for a whole minute while silly music played.]] The unused scene was recreated [[https://youtu.be/BsuH5UPaCYw?si=7911e1rprY0Yq7xI here.]]
* WordOfDante:
** Everyone in the fandom seems to be working on the assumption that the six human [=SOULs=] were children like the Human Child [[spoiler:and Chara]], even though there's no evidence of this in game at all.
** Similarly, everyone assumes that the Mystery Man and W.D. Gaster are one and the same, despite a complete lack of direct evidence--but to be fair, it's the best guess anyone has. The closest thing to official acknowledgement is the use of the Mystery Man on Gaster's card in Fangamer's tarot card set for the game, but even then, these are based on licensed-out [[https://dogbomber.tumblr.com/post/132310526044/here-it-is-the-full-set-of-undertale-tarot-cards fanart]] rather than being specially commissioned.
** The theory that [[spoiler:Chara is the game's narrator]] has never been explicitly confirmed, but the idea is so widespread and has been explored by so many fan work creators that it's almost universally considered canon.
* WordOfGod: Toby has made a series of tweets answering questions about the game or hinted at its lore in the past, a good chunk of which has since been deleted. He would [[https://twitter.com/tobyfox/status/802150493992714240?lang=en later state]] in 2016 that they were not canon, but the shitposty nature of the [[https://twitter.com/tobyfox/status/802147835244400640 two]] [[https://twitter.com/tobyfox/status/802148038072528896 tweets]] prior to it muddies things on that end:
** While it's already heavily implied in-game, in response to a user incorrectly stating that none of the six humans made it ''to'' Asgore, Toby [[https://web.archive.org/web/20160111003531/https://twitter.com/FwugRadiation/status/651827286514233345 clarified and confirmed]] that all of them got to Asgore and subsequently perished by his hand.
** When people were messing around with the "Fun" values, [[spoiler:particularly with Gaster]], he stated that the machine [[spoiler:in Sans' basement]] can never be fixed, no matter how much effort is put in. What happened with it [[https://web.archive.org/web/20151107003935/https://twitter.com/FwugRadiation/status/644973310481166340 is considered the happiest outcome.]]
* WhyFandomCantHaveNiceThings: When the game was still in its Kickstarter phase of development, one donor had given enough money to put a character of his into the game as an EasterEgg. The donor in question decided to use his fursona as the character, heavily toning his features down and renaming him to make him more suitable for the game. When people found out about the character, he generated a massive backlash for several reasons [[note]] namely the high amount of money the donor paid, the fact that the character was used in fetish art outside the game, and a perception of the character's inclusion as a self-promotion attempt[[/note]], which only got worse when anti-furry trolls jumped on the bandwagon. The overwhelmingly negative reception led to harassment directed at the donor; Toby Fox himself made a (now-deleted) tweet asking them to stop, but the donor nevertheless stepped away from both the furry and online art communities in the end.
* WorkingTitle:
** During the Kickstarter period, the game's title was capitalized differently, being seen as both "[=UnderTale=]" and "UNDERTALE". Eventually, it morphed into "Undertale" and stayed that way.
** "Heartache" was originally titled [[spoiler:"Joker Battle"]], fitting its original intent as [[spoiler:Jevil's battle theme]] in ''VideoGame/{{Deltarune}}'' (which was conceived long before ''Undertale'').
* WriterRevolt: Sony mandated, among other things, that trophies be in the [=PS4=] and PS Vita versions of the game. Toby Fox hated the idea of included VideoGameAchievements, as one of the major points of the narrative was deconstructing OneHundredPercentCompletion. So the [[https://undertale.wikia.com/wiki/Trophies trophies included]] were about half {{Effortless Achievement}}s (getting a small number of items, progressing through the main storyline regardless of your path) and the rest were donation to the Dog Shrine added to this version. All of them have essentially the same icon, making it comically obviously that as little work possible was put into them. Some of the trophies listed are:
-->1. Don't Worry, I Have Lots of Ideas for Trophies\\
2. Like Getting Items\\
3. Or Getting More Items\\
4. Help Me, I'm Out of Ideas
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!! General Trivia

* ''Undertale'' and ''Deltarune'' are MediaNotes/{{trademark}}ed by a company named Royal Sciences, LLC.

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