Developers Foresight / Undertale

There is a lot to discover in Undertale. Sometimes, it seems that there's not a single thing a player can do in this game that Toby Fox did not think of and account for, as the length of this page will attest to.

Due to the nature of this page, BEWARE OF UNMARKED SPOILERS.

Ruins

• If you reset and speak to Flowey again after meeting him once but before completing a Neutral Route, he'll react as if you just walked off on him the next time you start the game, foreshadowing the fact that he's aware of your ability to SAVE and that he retains his memories even if you LOAD to an earlier point.
  • After restarting the game two or more times, Flowey will get irritated, asking you if you have anything better to do.
• If you manage to reach the pair of levers and pull the unmarked one before Toriel tells you to pull a lever, you get a unique message.
  (Wow! You are superfast at being wrong.)
• After testing your independence, Toriel leaves you with a cell phone and goes on ahead, telling you to wait. Of course, this means that you have to continue through the Ruins alone to progress in the game, but if you actually do wait, you'll get a phone call from her every 5 minutes. She apologizes for taking so long until a dog steals her phone; the next few calls consist of the dog accidentally calling you while she's trying to catch up and get her phone back. Finally, she succeeds and apologizes to you again.
  • If you attempt to call Toriel while the dog has snatched her phone, but before proceeding to the next room, you'll get a special message.
  ("Somewhere, signals deflected by a dog.")
  • Toriel will also acknowledge this during her scripted calls to the player afterwards.
  • Later, if you do the piano puzzle in Waterfall and try calling her while you have the Annoying Dog in your inventory, you get a message about the phone ringing in your inventory, meaning the dog managed to steal it again. (This would also explain why you can't call Toriel after leaving the Ruins.)

• Attempting to call Toriel's cell phone while she's in the room has her call you a "silly child" and add that you can simply talk to her.
• If you go down to the basement without convincing Toriel beforehand that you want to leave the Ruins, she'll go to fetch you while making an excuse each time as to why she doesn't want you down there. Do it enough times in a row, and she'll make more and more excuses until she eventually becomes too annoyed to come up with another.
• During Toriel's boss fight, if you get really low on HP, she'll instantly abort her current attack and after that, only use an attack where her bullets will actively avoid you, making the fight nearly impossible to lose. However, if you intentionally hit her bullets when you have specific HP values that are barely high enough to prevent her from aborting her attack early and die as a result, Toriel's sprite changes to a shocked expression for a split second◊ before the screen blacks out.

• If you attempt to spare Toriel, she eventually stops attacking you as she starts to regret fighting you. But, there's nothing stopping you from selecting the "Fight" command during this, and doing so will instantly kill her. Toriel briefly remarks that "You really are no different than them!" before she dies.
• Flirting with Toriel over the phone will result in her writing it off as a joke. Do it again, and she will think your advances are endearing, but also a bit sad. Do it again after choosing to call her "mother", and she replies that you are "...an interesting child". She'll then talk about this during the Playable Epilogue of the Pacifist route if you were to talk to her. If you do these things in the opposite order, she'll have a subtly different reaction, with entirely different dialogue.

Snowdin

• When Sans offers to sell you Fried Snow, if you keep saying yes to his offered price, he'll keep adding more zeros to it until he's asking 50000G for it, far more than you will probably earn throughout the whole game. If you still say yes despite lacking funds, he says, "you don't have the money? hey, that's okay. i don't have any snow." But if you somehow do have enough money (almost certainly through cheating), he'll say something along the lines of, "wow, that's a lot of cash. that's why i'm sorry to say... i can't sell you this fried snow. it's got too much sentimental value."
• Normally, if you eat or discard a Snowman Piece, you can return to the snowman to tell him you lost it and get a replacement. However, if you eat it right in front of him, he'll be suitably horrified and refuse to give you any more. Continuing with a True Pacifist Route and then returning to him after triumphing against the final boss leads him to say that he knows "the truth" about you, despite what other people may think, and he will comment that "your heart is as cold as my butt." If you do this during a No Mercy run, the snowman will beg you to stop eating him. You can then continue to take Snowman Pieces without his permission until he's reduced to "a useless pile of snow".
• Using the "Gift" ACT on Gyftrot will have you give it up to 30 G. If you have less than that, the game will describe you giving it all the money you have. If you have no money (pretty much has to be on purpose at this point in the game), then you'll give it the gift of friendship.

• If you try to walk away from Papyrus' "invisible electricity maze" before you "solve" it, he'll call you out on it. This is noteworthy for being one of the few moments where he doesn't Comically Miss The Point if you choose to be a jerk.
• If you decide to backtrack after reading the word search puzzle, without going through, Papyrus complains that Sans made the puzzle way too hard. Of course, since the puzzle is unsolvable anyway, that's not entirely untrue...
  • Expanding upon this puzzle, if you do a Genocidal Route and check this puzzle before talking with them, Papyrus will be agitated by the fact you actually bother to check it out (as opposed to just walking through all the puzzles HE set up), declares that he is done with them and leaves. Sans would also comment that you must not be all bad if you like Word Searches.

• When Papyrus uses his "blue attack", he sends out a light blue barrage of bones that can be dodged by staying completely still, before turning your soul blue and sending out a bone from the left side of the box to demonstrate your soul is subject to gravity. If you get wise and move your soul towards the right before you're turned blue, that bone enters the box from the right.
• When you're in Papyrus' house prior to the date/hangout, he normally stays to the left side of the living room. Entering the kitchen makes him walk over so he can comment on what you're doing there, and exiting the kitchen makes him walk back to his usual position. However, if you walk in and out of the kitchen repeatedly, he starts moving faster in his attempts to keep up with you, with an audible "ping" noise whenever you toggle his movement. If you talk to him afterwards, he'll say that being a good host is a real workout. If you keep doing it until you hear another type of noise, he'll say that he can't feel his legs and thinks it means that he's a great host.
• At the start of the date/hangout with Papyrus, you can call up the interface early by pressing the 'C' key. After Papyrus reads that step in his guidebook, he'll say, "I think you already did that." Also, if you wait a while without pressing the 'C' key to bring up the dating/friendship interface during the date/hangout with Papyrus, he will decide to just go ahead without it.

• During the "inspection" part of Papyrus' date/hangout, Papyrus will comment if you try to move out of the screen, thinking you're trying to run away because you're overwhelmed by his cool vibes. (This also instantly teleports the SOUL back to where it started.)
• If you buy the Mystery Key from Bratty & Catty and try to use it on the secret door, the game gives a unique response:
  ''You used the Mystery Key. The door's lock clicks... as you fail to fit the key into it."
• On a No Mercy run, if you somehow have not killed Snowdrake by the time you get to Snowdin Town, the red text that shows up at a save point changes from a remaining monster counter to "That comedian...", hinting that you need to kill Snowdrake to maintain the No Mercy conditions. Fail to kill him before the monster counter reaches 0, and the No Mercy run will be aborted, with the text changing to "The comedian got away. Failure." ^{Programme note} This was bugged in the initial release of the game, resulting in a lot of misunderstandings about who exactly "that comedian" was and why they mattered. The above text describes the intended, post-1.1 behavior.

Waterfall

• The NPC who explains how echo flowers work is standing next to one, which means it will hear the explanation and repeat it. If you "talk" to the flower before talking to the NPC, it will have different dialog — which will in turn change the NPC's dialog, which means the flower will repeat that instead.
• If you backtrack to Papyrus's house after witnessing his meeting with Undyne, Papyrus will sweat about how unsuspicious a friend he obviously is.
• A few screens after Papyrus meets Undyne, Papyrus will call you and ask what you're wearing. There are a few different ways this can play out with normal player behavior (ie, tell the truth and he'll have a piece of dialogue, lie and change your armor and he'll have a different line, etc), but even if you cheat to give yourself anything unavailable at that point in the game yet, Papyrus says that he'll tell Undyne you're wearing a "bepis", a fake Running Gag swear word of Toby and fellow indie dev chess.
• Normally, using the "Touch" ACT on Woshua will do nothing other than provide flavor text depending on whether it's cleaned you or not. However, if you Hug a Moldbygg in the same battle (something that's normally very counter-productive), touching Woshua will make it flee the battle in terror.

• If you linger around long enough in the first encounter with Undyne, she'll start hurling spears so quickly that they become a nigh-impassable stream of them, and the Monster Kid will remark that something's gotten her extremely frustrated when she mistakenly grabs them instead of you.
• You can win the Thundersnail minigame if you "encourage" at just the right tempo, but you win 9 gold even though you paid 10 to play the minigame (they have to make a profit somehow). However, if you tie or get an extremely close second place with another snail, you get 30 gold instead because your snail thinks it won and Napstablook doesn't want to hurt its feelings.
• Skipping the piano puzzle when it first becomes available, getting the cell phone upgrade, returning to do the puzzle and get the artifact, and trying to put the dog in the Dimensional Box gives the message, "The box is aclog with the hair of a dog."
• There's a hidden message for the occasion of the player somehow removing the Annoying Dog from the inventory without triggering the cutscene of it absorbing the artifact (only possible via debug mode).
• If you take an umbrella and backtrack, you can get some unique comments from others:
  • Sans admires it and says he wished he had one since he's "getting a nasty burn from all this sun".
  • The Nice Cream Man offers you a discount.
  • Goner Kid, a character that can only be talked to if you have the right Fun value, will state that they appreciate the thought of you bringing an umbrella to them.
• As stated on the main page, Flowey stalks you during most of the game and if you backtrack at specific points of the game, you can quickly see him retreating underground. Outside of the Genocide route, if you do so in the vertical room with a several echo flowers and the sign about the "fallen angel", he'll leave a message to the echo flower closest to him, either taunting you about how Toriel will forget about you and find another child to take care of or how she's trapped in a cold, dark void with nobody to rescue her, depending on whether you killed her or not.
• If you watch Monster Kid fall off the bridge, Undyne will jump down to grab them and fall down the chasm. Monster Kid will then remark that Undyne was injured by the fall. Indeed, if you do this and go on to fight Undyne, she will start the fight already having lost a bit of health.
• At the start of Undyne's boss battle, if you do the opposite of what she says and get hit by her first attack instead of blocking it, she'll explain more directly what she meant by "face danger head-on" and repeat the same easy attack on the next turn. Continuing to get hit by this attack will make her more and more frustrated, until she finally unleashes an attack that's hard for Neutral, but not Genocide.
• If you run away from Undyne's boss fight, when you reach the "Welcome to Hotland!" sign and Papyrus calls to say you should all meet to hang out, you can double back and go to Undyne's house to find Papyrus waiting. He'll commend you for coming so quickly, but then notes that Undyne isn't home yet (she stops chasing you after the SAVE point).

• Within the game's debug mode (only accessible via cheating), Undyne's so determined to get you, she's immune to the debug command meant to make monsters instantly able to be spared within both her normal and "Undyne the Undying" form (except during her "date").

Hotland

• After Undyne's chase ends and she collapses from the heat, the game will call you out for being sadistic should you pour all of the water onto the ground instead of on her.
• The water cooler visibly empties and will eventually run out if you keep taking cups of water and dumping them on the ground. The NPC that appears after the first time you visit that screen also changes his dialog to comment that they have a private pool now. If you make the puddle big enough and then come back during the True Pacifist epilogue, there will be a small tree growing in the puddle.

• The second floor of Hotland is blocked off by a force field on a Genocide run, thus making it impossible to access the hidden Art Club room (where the Optional Boss So Sorry can be encountered at the date and time listed on a sign). However, if you access it on a Genocide run anyway (such as by modifying your save file), the sign says that Art Club is cancelled, and So Sorry never appears.
• Muffet notices if you eat a Spider Donut or drink some Spider Cider during the fight with her, letting you end the fight non-lethally as early as the first turn. She also skips the fight entirely if you buy one of the 9999-gold items from her bake sale a few screens earlier, which you likely won't be able to without unreasonable amounts of grinding or an edited save file. If you obtain one of the Bake Sale items through cheating, she scolds you for being stingy, but end the fight regardless.
• Staying during battle while Muffet's sparing you (in addition to not attacking her) will cause her to comment about this.
• Looking into the game's source code and checking the part of Mettaton's quiz where he mentions Alphys naming programming variables after her crush reveals an in-character comment from Alphys stating she won't reveal who said crush is.
• During the bomb-defusal sequence, despite it being a Timed Mission, you are still able to use the Check option on all of the bombs, each having a unique message based on the object.
• If you happen to be standing too close to or on top of Mettaton after the opera parody, he'll move away from you before he presses the button to drop you into the dungeon.
• If you load after Mettaton's opera, he will notice you will have a bored expression and immediately asks you if you want to skip the scene.
• Mettaton's tile puzzle can be solved◊. Doing so will lead to extra dialogue. Also normally, when the puzzle ends, Mettaton reveals that he's the enemy you have to fight for stepping on the green tiles. If you didn't step on any green tiles (most easily seen by simply not doing the tile puzzle at all), he'll be at a loss for words before finally acknowledging it... and fighting you anyway.
• If you don't die at all before having dinner with Sans at the MTT Resort, he'll mention it in the conversation (normally, he questions if you'd died before).
• At the MTT Resort, one of the hotel rooms will give you the Hush Puppy if you give it a Hot Dog. However, it also accepts related "dog" items; it will duplicate the Dog Residue and will "absorb" the Dog Salad. Additionally, attempting to give a Hot Cat (an item only gotten by buying more Hot Dogs at Sans' stand) will anger the resident.
• Mettaton EX has a number of responses during the "essay portion":
  • If you insult him, he says that the essay should be describing him, not you.
  • Typing in any swear words causes him to complain that you should keep his show appropriate to all ages (before telling you to stand still so he can kill you). This also applies to "bepis", a word commonly used in Toby's online social circle as a fake swear word.
  • There are a number of unique responses if you compliment his hair, various body parts, personality, voice, or dancing skills. You can get the highest number of rating points by commenting on his legs, which he proclaims is "The correct answer".
  • Telling Mettaton you love him will cause him to comment, "What a touching confession! I'll add it to the pile."
  • Simply saying "Mettaton" will make him compliment you on your brevity.
  • If you mention Toby, he doesn't have any idea who the hell that is, but he thinks he sounds sexy.
  • If you type gibberish, he'll remark that you did a fine job, even if he doesn't understand a bit of what you wrote.
  • Typing nothing at all will have Mettaton remark that you must be speechless.
  • Mettaton will also have a suitable response should you type in only one character.
  • On the other hand, if you type a ridiculously lengthy response so that the text won't even fit in the dialog box, he's impressed and thinks you should write a book.
• During Mettaton EX's boss fight, if you use the Mystery Key (the key to Mettaton's old house at the Blook Family Farm), he'll "pretend it isn't there".
• Sparing Mettaton during his boss battle and then coming back to Alphys' lab before the epilogue will let you see Mettaton's EX form in Alphys' room, undergoing repairs. Given that you can only enter Alphys' lab post-Mettaton battle after her date in the pacifist route, the window of time to see him there is very small — especially since you're supposed to head to the True Lab after the date, where upon entering, you'll be unable to go back until the Playable Epilogue.

Final Area & Endings

• If you edit your save file so that your "kill count" is higher than than it should logically be (like you being at level 1 instead of anything higher), Sans will notice and call you out on it instead of actually giving you your judgement.
  Sans: come on. really?
  • Normally, you'd only reload your save and hear Sans' judgement again when you're trying to get him to give you the key to his room, but he also has a large number of speeches where he judges you himself depending on how high your LV is that you can only see if you try to get the key but won't qualify for it since you're not on a Pacifist run.
  • If you listen to Sans' judgement, then reload your save and kill enough monsters to increase your level before being judged again, Sans will notice.
    Sans: did you go through and kill someone... just to see what i'd say about it? wow. you're a pretty gross person, huh?
• If you keep the slice of Butterscotch Pie from Toriel until the end of the game and eat it during the fight with Asgore, the smell will remind him of Toriel, lowering his attack and defense. The same trick also helps Asgore and Toriel remember you while you are trying to SAVE their lost souls. Although it's normally impossible without hacking due to Hard Mode ending right after Toriel's battle in the Ruins, the "Snail Pie" (which is exclusive to the Hard Mode) also works for the exact same reason.
• Sans's phone call at the end of a Neutral-Pacifist run changes based on a number of criteria, such as: which major NPCs you spared or killed; which ones you befriended on the Pacifist route; whether you killed a lot of encounters but didn't trigger the No Mercy route; whether you started the No Mercy route but aborted it by sparing a major NPC. The most obscure condition might be befriending Undyne, which is only possible on a Pacifist run, then killing someone afterwards (which itself has two variations depending on if that person was Mettaton or a normal monster). He may also start the call with an additional message for completing one of several challenge runs. From easiest to hardest: have a Snowman Piece, never saved, wearing the Bandage, never used healing items, and Bandage/no healing items/level 1. If you have killed Papyrus, Sans will skip all this. Finally, if you get to the ending with invalid Event Flags, Sans gets confused, Dogsong starts to play, says "this is actually some sort of error-handling message" and asks you to tell the developer about how you got there... unless, of course, you're just a dirty hacker.

• After either completing a True Pacifist Route or defeating Sans, there are no normal encounters left, and it is impossible for the player to take damage from monsters on the overworld. The game is also programmed to disable the lasers on the Hotland conveyor belts to prevent the player from intentionally running into the lasers and dying. The laser control switch at the beginning of Hotland even has a unique message:
  * (Didn't work.)
• During the True Pacifist route, it's possible to drop Undyne's Letter. As the game points out, this seems to defy common sense; luckily, you can go back to her and ask for a replacement. This nets you some extra dialogue, and the replacement item "is too powerful to throw away. It gets the better of you." You can also attempt to open the letter right in front of Undyne despite her explicitly telling you not to peek. Her reaction is priceless: "OH MY GOD!!! I'M GONNA FREAKIN' KILL YOU!!" If you try to drop it in front of her, she'll stop you and insist that you're supposed to deliver it instead. If you try to use it while standing in front of Papyrus' mailbox, she tells you that she told you to deliver it to Alphys, not Papyrus. And finally, you cannot sell it at Temmie Village, with its price being "NO! G".
• The Hush Puppy item is said to be able to neutralize dog magic. True enough, using it in the battle against Endogeny is one of the few ways to easily end the fight against it, the other being to USE the Stick.
• During the first phase of the final battle in a True Pacifist run, you can spawn a special healing item in your inventory called the Last Dream. Any leftovers stay in your inventory after the battle, have a description, and can be sold for a premium in Temmie Village.
• During the cutscene where Photoshop Flowey taunts you with his Save Scumming powers after you've successfully defeated him, he will react if you repeatedly ram yourself into the bullets surrounding you while he's gloating, getting annoyed at you and telling you to stop it.
• Dying to Photoshop Flowey repeatedly causes his dialogue before the fight to change. After enough deaths, he spells out for you that his godlike powers have erased the world and that there's nothing left, no happy ending to fight for. After a few more, he starts to wonder if you're letting him kill you on purpose, but decides that killing you is so fun that he'll gladly take it (even if you're a sicko). Eventually, he just starts asking you "Don't you have anything better to do?" If you close the window at any point during his first phase, then the next time you start the game up, he'll admonish you for trying to run away.
• In the True Lab, the room with the fridges is completely fogged up and impossible to see in until you turn on the air conditioning. If you explore the room in its fogged-up state, each fridge gives you an unique description. Interestingly enough, the only fridge that actually gives a fitting description ("a cold, rectangular object") is actually the one that's not a fridge at all but an Amalgamate disguised as one.

• It's possible to beat the game without SAVING at all, not even once. While difficult, this nets the following:
  • Photoshop Flowey will mock you for not having created a SAVE file instead of bragging that he destroyed the SAVE file.
  • In Neutral Endings, Sans will suggest you to SAVE.
  • During the final boss of the True Pacifist Route, the part where you try to reach your SAVE file only to find that you're overpowered is replaced. Instead, Frisk considers saving over Asriel's file to defeat him, but realizes that without their own SAVE file, they can't. The game will also silently save a file for you, to prevent the happy ending from being lost when you quit.
• The game creates a special file^note system_information_962 if you've completed a Genocide run telling the game that the world is "destroyed" and starting the game afterwards leaves you staring at an empty void. If you sell your soul to the fallen child after being confronted with said void, it creates another file^note system_information_963 that tells the game you did so, tainting all your future playthroughs unless you delete it. What makes this a trope example is if you're playing the Steam version. Steam Cloud seemingly doesn't do anything...unless you complete a Genocide run or trade the soul to restore the game. This means that even deleting the game and every trace of it from your computer isn't enough to escape your actions. Unless you think to check your Steam Cloud (which you likely have little reason to do), the game will eternally be left in a soulless, corrupted state. And it's all your fault.
• Three closely-related aversions from the Genocide route:
  • At the end of New Home, Flowey will suddenly realize that you're both willing and able to kill him for real, and declare that he's changed his mind about all this. Resetting the game at this point is not acknowledged in any interesting way; Flowey will give the same "Really?" response he would've for resetting the run any other time.
  • Before the fight with Sans, if you chose not to equip the "Real Knife" the animation for the finishing blow against Sans and later the murder of Flowey will still play as if you have the knife equipped.
  • After defeating Sans and delivering the final blow to Flowey, you will meet the Fallen Child. While the game normally prevents you from quitting, like if "Chara" was preventing you from escaping, if you close the game any other way, the game will not recognize you for doing so, averting the final fate of the route.

Replay Value

• If you keep reloading a save just before the Asgore fight and repeatedly watch the outcome, Flowey will wonder if you want to befriend him, and call you pathetic (assuming you spared him after his Photoshop fight). Doing it again will net you some extra dialog after the ending.
• Flowey's dialogue changes if the player starts a No Mercy run but restarts before finishing. Upon meeting him in the Ruins, he says something along the lines of "Really?", leaves without engaging the tutorial fight, and doesn't show up after the fight with Toriel. At the end of a Neutral run, sparing him normally has him hint at how to get a better ending. In this case, he'll complain that the two of you came so close to ruling the Underground together, only for you to throw that chance away.

General / Miscellaneous

• When the game was first released, the internal resources of the game (music, text, graphics) included hidden messages asking dataminers not to post data files online. The 1.001 update replaced them with nonsense messages, permitting the dataminers to publish the files.
• The website's source code has a hidden message.
  What are you doing? Looking for secrets? Don't put your nose where it doesn't belong. Or you might learn something you DON'T like... Hee hee hee.
• When the game asks you to name the fallen child, certain names — including names of NPCs — will elicit a reaction, and some will tell you to knock it off. Examples:
  • "Frisk" is the Player Character's real name, as revealed at the end of the True Pacifist route. Since it wouldn't make sense if it is also the Fallen Child's name, using it enables a joke Hard Mode that only lasts until the end of the Ruins.
  • "Mercy" and "Murder" are thought to be a little on-the-nose.
  • "Gaster" resets the game because a certain Dr. W.D. Gaster is lost in pieces through time and space...
  • "Chara" results in the game stating it is "the true name".
  • Naming the Fallen Child after certain characters (like Sans, Flowey, or Toriel) would cause them to outright prevent you from using their names, forcing you to pick another name. Conversely, naming the character after certain other characters (like Papyrus, Woshua, Mettaton, etc.) will net a comment from them stating that they will allow you to use their names. (In the case of Papyrus and Mettaton, this is because the six-character limit means you can't use their names exactly without hacking the game; their reaction comes from using similar names [i.e. "Papyru", "Mett", "Metta"]. For others, this is because the characters are minor enough to not cause story problems via violation of the One-Steve Limit.)
  • "AAAAAA" will net a comment about the name being "not very creative".
  • While Alphys is among those who refuses to have their name be given to the Fallen Child, if you remove the "s", she'll instead react in confusion before allowing you to use it.
  • "Drak", "Gigi", and "Gugu" causes the "silly" shortened form of "Face Steak" to be "FSteak" as a reference to Gigidigi/Hiimdaisy's Persona 4 comic.
  • Finally, if the game catches you editing your save file in order to change your name (for example, if it's longer than 6 characters), a message appears in the empty space next to the status bar that reads, "Easy to change, huh?"
  • Here is a video with all the name Easter Eggs, including several of the monster names.

• Your starting weapon is the Stick, which is placed in your inventory if you equip another weapon. It's surprisingly versatile:
  • Snowdin is filled with dogs. Every one can be spared immediately by playing fetch, with different dialog afterward from the ones that talk. It also works during Papyrus's fight (where he'll fetch it just like the dogs would) and against a special dog found near the endgame. (Using the Stick against Doggo, Dogamy, and Dogaressa is the only way to see their white text credits.)
  • It gives you gigantic ratings during the fight with Mettaton EX (as using it during his fight would cause him to do a short skit involving catching the Stick with his teeth).
  • One of the final guards, Madjick, will think you're a fellow wizard if you use the Stick like a wand. Thus, you can Spare him right away if you use it.
• If you edit the save file so that you are in Hard Mode and within areas from after Toriel's house at the same time (something normally impossible due to the Annoying Dog stopping "Hard mode" after her boss fight), the game will react by fading the screen to black, making it impossible to continue.

• Looking into the game's resources to reveal the enemy list shows Dr. Gaster programmed into the game as an unused (and possibly incomplete) enemy. Each and every one of his stats, including the number he's internally referred to as in-game, is 666 (or its variants). This is all because Dr. Gaster is specifically designed to be a character that's Dummied Out.
• The game has a "serious mode" that activates during a few dramatic boss battles. To avoid spoiling the mood with jokes, this changes the names of all items with silly names on the combat menu to less silly ones (for instance, renaming the Butterscotch-Cinnamon Pie "ButtsPie" to "Pie", or the Legendary Hero Sandwich from "Leg Hero" to "L. Hero"). It also removes silly sound effects after using certain items, and replaces the Overly Long Gag effect of the Instant Noodles with eating them raw for a much stronger heal.
• The Annoying Dog can only legitimately be found within your inventory while you're in the treasure room past the room with the piano. However, should you somehow end up having it in your inventory while outside said room, not only does it have a shortened version of its name (complete with a "serious" variant), it also goes for a high price at Temmie Village.
• The Genocide route-only item the "Real Knife" is programmed to make most monsters instantly spareable. As the only other monsters you can fight by the time you get the Real Knife aren't spareable by any means, the only way to see this reaction is via editing your save.^note The technical reason this works is that having high enough AT makes monsters instantly spareable, and the only in-game way to get your AT that high is the Real Knife's 99-point boost.
• The out-of-the-way healing item "Abandoned Quiche" has a unique drop message along the lines of "You gently place the quiche down and tell it you'll be right back." This is continuing along with the Running Gag involving Toby finding a quiche inexplicably underneath a bus stop bench.
  • If you try to take the quiche when your inventory is full, the game will say "You're not ready for the responsibility" in addition to telling you that you're carrying too much.
• The official Toriel plush has a secret, though one would need to be pretty heartless to discover it: hidden inside of the plush is a Heart/SOUL, and the only way to get it is, of course, to cut open the plush.

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