Toby had planned to make the color of the grid in the background of battles change depending on the area, as opposed to always being green as it is in the final game. This was planned for Deltarune as well, but he forgot to include it.

During the battle against K. Round on the Great Board, if you wait long enough when it initiates its self care attack for the first time, you get your turn back before it heals itself. Which leads to it healing on the next turn and breaking the dialogue box.

All the code objects for the soul sections of Noelle's house are prefixed with "homealone", which we know now is a reference to an idea Andrew Hussie had for the section as revealed in the Winter 2025 newsletter.

The beginning of the 2025 Winter Newsletter references this unused prophecy panel.

Contrary to what was said in the Summer 2023 Newsletter, the Winter 2025 newsletter states that the track "TV WORLD" was composed in 2015. It is unknown if this is a typo or a correction.

When the Annoying Dog picks up the bone after the secret climbing race in Chapter 4, it uses the "item obtained" effect from The Legend of Tenna. This is possibly due to the Annoying Dog being an "avatar" for Toby similar to how the heroes have their avatars in game.

When starting Chapter 3, Ralsei's "Guts" stat increases to 1, but drops back down to 0 when starting Chapter 4. This is the only time this stat is lowered.

If you do poorly enough during the Boards it is possible to get a "Z-Rank" which is the lowest rank you can obtain. It has its own changing room and also allows you to play the original game.

In the scene where Susie talks to Tenna alone, Tenna says that he "LOVES" Kris. This is translated to "は！？　ワタシは　クリスLOVE ですが！？" in the Japanese translation, with LOVE spelled in English.

Ralsei's response to saying you want to take Noelle to the festival is different if you did the weird route.

During the song sung by The Muppets characters Miss Piggy and Rowlf the Dog during The Game Awards 2025, Undertale was briefly mentioned in the lyrics.

When working on Mettaton EX, Toby was worried about the difficulty of the fight. Because of this he made a separate program that only had the battle in it, and he gave it to his friends to see how many tries it would take to beat him.

When Deltarune first released, some people didn't know what to do when they saw windows defender, so Toby posted a very helpful tutorial.

The song "H3@RT$ W3RE M3@NT T0 F7¥" by Childish Gambino (aka Donald Glover) uses Flowey's voice blip as a sample.

When Toby was reflecting on people asking him how he makes characters, Toby shared his belief that the most important thing to making characters is to make a story with them first. "Purpose and function first, personality, etc, later."

In Chapter 3 when Susie throws Ralsei into the ocean during Board 1, Tenna says "Ralsei plush! Fill him with liquid, throw him against the wall!". Which is a reference to the meme of the person who filled their Webkinz plush with milk and slammed it against the wall.

If you fall 10 times in the Ruins room where you need to avoid the red leaves, all the tiles will allow you to walk on them. And at the top right there is a wall without collision that allows you to walk out of bounds.

In battle Glyde has a 1% chance to not attack.

When playing the "Battat" Mike minigame, IMAGE_FRIEND has a chance of appearing when you make a level 40 or higher cat hit a meteor. And when touched FRIEND gives a random amount of points between -50,000 to 300,000.

If you backtrack to this wall after unlocking the Study, you have the option to hug the wall, and you can do this every time you leave and enter the room.

As revealed in the Undertale Art Book, Asgore was going to have a special attack but it wasn't added because it would make the fight too difficult, and also because his spear wouldn't fit on screen.

If you interacted with Starwalker in Chapter 1, as well as played the monster Movie minigame, Starwalker can be seen helping Kris move Susie around in the "Protect Ralsei" minigame during Tenna's fight.

In the launch version of Deltarune Chapter 4, if you stood in a specific spot in the Second Sanctuary and pressed "B" on your keyboard, you would get softlocked. This has since been fixed.

The Playstation 4 and Playstation 5 versions of Deltarune have an exclusive ending speech from Gaster if you manage to beat all 4 Chapters without obtaining any of the trophies.

On the Desert Board, there is unique dialogue from Tenna if you discover the mailroom but don't enter it

The text you get when interacting with this pile of hay in the TV World changes if you have the Dealmaker equipped.

Level Select Podcast, a video game podcast by Fred Wood, features a theme song made by Toby Fox.

The official Youtube and Spotify uploads of the song Crumbling Tower, where it is listed as the 66th track, have an audio glitch in them at the 2:19 mark where the music cuts out and is replaced with static briefly. This does not appear in game or on the bandcamp version.

Upon completion of the Sword Route, the "free" SMILE option from the vending machine in the S-Rank room becomes sold out.

In Chapter 3, during the "What's Kris's favorite food?" And "What was Asriel's favorite character in Smashing Fighters?" Quiz questions, when you are hovering over an incorrect answer, Kris will cough and attempt to switch it to the correct one.

There is an unused variant of the song "Digital Roots" which is sped up and has the wind ambience removed. The filename, "spamton_house" indicates it was meant to be used in the room outside Spamton's shop, which in the files is named "room_dw_city_spamton_house"

Loox was the first enemy in Undertale to be designed by Temmie Chang.

For Undertale's 10th Anniversary, SQUARE ENIX is collaborating with UNDERTALE. With a new album from SQUARE ENIX MUSIC's Chill Arrangement series featuring music from UNDERTALE, as well as some new merchandise.

In the Dark Sanctuary this room has a chest hidden in the darkness that you can interact with. Although it doesn't give you anything.

In an old Twitlonger, Toby stated that his maximum time willing to spend on a project is 7 years. However obviously this isn't true anymore as Deltarune is over 7 years old now.

Two months ago, Toby Fox's company "Royal Sciences LLC" sent a couple of DMCA takedown requests towards some fan/piracy websites for copyright infringement.

In the Switch 2 version of the Mike Room, if you aren't already using the JoyCons a switch will appear on the wall letting you change controllers.

Also, the books in the T rank room are a reference to the Japanese game "Roly-Polys Nanakorobi Yaoki". The song in this room, titled "KING OF ROLYPOLY", also references the game.

The T rank room is different depending on if you visit it after Board 1 or Board 2.

The game "Contract Rush DX", a game by Figburn, a developer for the fangame Undertale Yellow. Features a cameo from the Undertale Yellow character Mo.

In Chapter 4, the dialogue option "I'll never play again" changes depending if you are on the weird route or not.

In the indie game "Mago", the characters Dalv and Mo from the fangame Undertale Yellow can be found as cameos.

When Tenna switches through channels rapidly at the end of his fight, on release there was a bug for PC caused by using certain GPUs that would display texture data on the screen instead of the intended static. This has since been fixed.

In the credits of Stray Children, the names "December Holiday" and "dog" are right next to each other as voice actors.

There is unique dialogue from Ralsei if you check the Titan while its shield is down, and unique narration if you do it again.

On December 24th, 2012, Toby posted the song "tomorrow's holiday" to his tumblr, this same song would later be used in Omori as the song "Merry CD"

Yoshiro Kimura, creator of the game "Moon: Remix RPG", said that playing Undertale gave him the courage needed to make his new game, "Stray Children".

Inside the Fangamer Tenna plush is a pipis.

On July 3, 2015 Toby uploaded a file called 67.wav to his website.