When you use the OldMan act on Balthizard for the first time, the Old man comes in and asks how the fight is going and also asks "Ya winning, kids?", a line that references the "Are Ya Winning, Son?" meme.

The dialogues in Undertale and Deltarune about bad and dangerous combinations is likely a reference to some obscure dialogue in Earthbound where a cow says "Cows and carrots?... That's a nutty combination!"

The Lode Stone item in Deltarune has several unused flavor texts as well as unused equip dialogue.

When you investigate Noelle's computer, Susie can comment on a "green dog puppet" wallpaper. This is likely a reference to a SiIvaGunner upload of Field of Hopes and Dreams, where Noelle wears a hoodie themed after GIR from Invader Zim, which became popular in the fandom.

When using the "Rival" act against Bloxer, there is a bug where Susie's talk sprite stays on screen when the narration says the enemies became tired.

In the original Survey_Program release of Deltarune, Jevil didn't give you a shadow crystal upon beating him because they didn't exist yet, shadow crystals only got added with Chapter 2 in the Chapters 1+2 Demo.

There is a visual bug when Tenna transforms all your commands into the ACT option during the Doom Board, for a few frames before everything changes into ACT, Kris loses the defend option but gains the magic button in the menu.

In Homestuck: Beyond Canon, a list of possible Kind Abstrata (weapon types) is shown. One of these listed is "anyngdogkind", a reference to the Annoying Dog.

Unlike the egg rooms in Chapters 1 and 2, which have a 1/50 chance to send you to them on the screen transition for their respective room. You will always get the Chapter 4 egg room after 8 attempts.

During The Roaring Knight's Box Cutter attack, getting hit by the slash itself will always leave you at 1HP. And if the target is already at 1HP, it shows up as a "MISS."

When you spare Tsunderplane its sprite becomes smaller and loses its hat, and moves to the top left corner of the screen, which the official Undertale FAQ claims was intentional. Toby later admitted this was originally a bug, but he kept it in due to it making him laugh.

As of Chapters 3 and 4, the HP that Kris has in the Dark World is reflected in the Light World, and vice versa. Although it isn't possible to ever die in the Light World because of this. Additionally, Save points in the Light World do not restore HP.

Seam has different dialogue when you talk to them in Chapter 4 depending on if you beat the Knight with or without obtaining the Shadow Mantle.

The song "Who...Are You?" from Final Fantasy VII is incredibly similar to Gaster's theme and likely served as an inspiration for the song.

If you defeated Jevil and talk to King in Chapter 4, you have an option to talk about him. King reveals that Jevil was the one that told him about the Knight and his future rise to power, and in another dialogue option it's implied King has never met the Knight.

If you start your save file in Chapter 3 without using completion data from the previous chapters, the Thrash Machine that appears during Tenna's quiz will be randomized, and the incorrect part is a fake piece that the machine can't be created with normally.

If you slay one of the Titan Spawn and purify the other, the stat Kris receives says "Purify: 2" as it takes priority over the slain stat.

Alphys's name is the Japanese translation is "Alfie", which was Toby's own suggestion. He says he was not aware of the Japanese band The Alfee at the time, and shows slight regret over this, as he wanted the names of his characters to be unique to be easier to search for.

In the animated music video for the song Muri Muri Evolution, the opening theme of the anime "There's No Freaking Way I'll be Your Lover! Unless...", there is a game over scene that references Undertale.

The song "O Chefe" by the Brazilian artist D$ Luqi samples Gaster's theme.

The Shuttah that can be found on the boardwalk in Chapter 3 has dialogue that changes depending on what you have done so far, specifically talking to the Bibliox in "Nowhere" and if you collected the egg.

If you read the SheetMusic without having watched the grand piano cutscene, it will open the door in the study. It also stops Fireplace from playing while in the study.

In the trailer for the Japanese film "君のクイズ" (English title: Your Quiz), a brief scene of the ruins from Undertale being played on a TV is shown.

When praying for Noelle in the Church during the Weird route, Kris will kick the candles.

If you take a picture of Rouxls in the Island Board, you will lose a point, this only happens once.

By just repeating the BreathAll ACT, you can spare all Rabbicks on the field at once.

There is a cutscene where Noelle mentions watching a horror series by a guy who gets mad at video games where a bucket comes to life. This is a reference to the Cinemassacre shows The Angry Video Game Nerd and Board James. Specifically an episode where a toy bucket comes to life and attacks James.

In Japan there are gacha machines that contain keychains of certain Deltarune characters. On the Jevil figure it appears that he is wearing a mask to cover his face. And although merch is not canon, the text "Toby Fox 総監修" indicates this was supervised by Toby fox himself.

Undertale was deemed Japanese people's 46th favorite game of all time in a survey covered by Japanese variety TV show "Ariyoshiiieeeee! – Oh yeah! Wanna play games at your place now?"

Toby fox owns a tumblr account called "Ponyprincess", which was mostly a side blog where he just posted whatever. Although it wasn't very active and didn't last very long.

If you mash text away at the maximum of 30 textboxes per second (done here with a mod), the lighting during the Chapter 2 Rouxls encounter will stay dimmed from the previous cutscene.

Deltarune tracks if you did the pasta easter egg with Lancer in Queen's mansion during Chapter 2. In an unused backstage room in Chapter 3, if you did the easter egg he has an extra line of dialogue.

Ghost Fight was originally made for a compilation of Super Nintendo-styled music. And as a fan of the game OFF, Toby felt that assigning a ghost character a jazz song would be a fitting combination. Though Toby doesn't think Napstablook has anything to do with Jazz.

The "free" SMILE you can purchase from the S-Rank changing room vending machine is likely a reference to the free smile available on the Mcdonald's Japan menu. It could also be a reference to Warioland 4, which similarly has a free smile that references the Mcdonald's free smile.

Anamanaguchi, the composer for the game Scott Pilgrim EX, dedicated the song "United By Fate" to Toby Fox. Anamanaguchi is also the band that inspired the song Hopes and Dreams, and the band even did a cover of it in 2022.

Stealth from the headcannon team built a Deltarune prototype in their methyl engine back in 2019 in response to Toby asking for help to build a team, Toby decided to stick with gamemaker so it was scrapped, but payed Stealth for his time.

During the cutscene where you have tea with the Old Man, after Susie leaves there is unique dialogue if you are on the weird route, and an alternate scene with the apple if you refuse to drink the tea.

The unobtainable TwistedSwd weapon also increases Kris's damage against the Titan and Titan Spawn just like the BlackShard does, despite lacking the SlayDark effect in its description.

There is a bug in the Hammer of Justice fight where if you use the "Glowshard" item after losing the fight once and starting it again, Susie's face sprite will appear as if she was speaking during the attack. This only lasts for one turn.

In the Undertale game files, there is a mockup design for Omega Flowey in which there is a pink soul.

The icon for Chapter 3 in the chapter select menu changes after you first finish it. The original icon resembles Tenna, while the alternate one resembles the Knight.

When using Dual Heal during the Titan boss fight, there is a mistake in the text that says "Susie and Susie cast DUAL HEAL!" instead of Susie and Ralsei.

Después de la batalla por el Manto Sombrío, puedes hacer que HERO_SWORD intente atacar a Susie, lo que provoca una nueva interacción entre Kris y Susie.

Most of Tenna's "funny text" is stylistically similar between both English and Japanese versions of the game, however the "Susiezilla" text is styled entirely differently in Japanese. With the Japanese version closely resembling the Japanese logo of the original 1954 Godzilla.

If you had Susie eat moss with Kris in Cyber City during Chapter 2, there is alternate dialogue when checking the hay in TV World's second cowboy minigame room.

Toby originally had planned more Royal Guards to appear in game, and thought of "insane evil situations" that would occur when you killed them. One of the ideas Toby told a friend about which did not like it at all, leading him to scrap it, which he says is for the best.

When Susie asks the "Gang" to pose together at the start of Chapter 4, she is supposed to mention the team name if one was chosen in Chapter 1. But due to a coding error she only says it if "The Fun Gang" was chosen and reverts to the default name of "Gang" for any other option.

In the files for Chapter 4, there is an unused Titan Spawn sprite that features Everyman's head and neck. It shares the "darkshape" part of its label with the other Titan Spawn. There is also unused code tracking if Everyman appeared in the cutscene just before the Titan fight.

The K-pop idol Gehlee has recently gone viral for publicly selfshipping with Jax, but revealed that Jax wasn't her first obsession with a character and that Sans was her first love, more specifically Underfell sans.

If you press "P" on your keyboard in the Dark Sanctuary money fountain room, it splits the waterfall and reveals the secret path even if you haven't donated money yet, this also works in the Second Sanctuary version.