In Pink's boss fight, if her song has the lyrics playing in the attack where she is on stage, she will lip sync to them.

In the Seth & Aqua fight, if you get hit by the duck attack when it would cause a game over, the duck uses a muscular hand to crush the soul.

During the segment where you design an outfit for Susie, one of the shirts, the "Rip Jacket", has a description that references the channel SiIvaGunner, as their videos are referred to as "high quality rips". Alongside that, there is a hat named silk that references Silksong.

Aqua's line about "the origin of fire" is likely a reference to a mistranslated line from Harvest Moon.

The credits of Chapter 5 have an unused line of text that says Chapter 6 is coming out in "20XX"

In April 2021, Toby posted a bunch of sprite data from the obscure Japanese game "Jingle Cats (PSX). Among these sprites was "Binky", one of the cats in the game. However Toby had changed it's colors from the usual light brown palette to a bright pink and yellow.

In battle, Floradinns have a 3% chance to say "Do it for the vine."

On the Weird Route in Chapter 5, if you interact with Ice-e's Susie makes a comment about pretending to work there and taking pictures standing on the salad, a direct reference to the burger king foot lettuce incident from 4chan.

Also about splendidland, Green from chapter 5 is based on her.

The Flavortext for the TrainPlan evidence in Chapter 5 is a reference to Mickeys dick smasher, a joke ride idea made by splendidland, who made designs for darkners in Chapter 2.

The previously unknown "annoying" song you could play on the Grand Piano in Chapter 4 is actually Flowery's theme.

Yellow very clearly references Woody from Toy Story, not only visually looking similar, but also with the line "I'm Yellow, Howdy howdy howdy!", and "Yellow's finest hour...!" referencing "Woody's finest hour"

In the S rank changing room, when you approach one of the stars on the wall it will fall onto the ground and make a spat sound effect.

Toby also suggested the name Cronus for the character Eridan Ampora, which later got reused for Eridans Dancestor.

Jens Bergensten, CCO at Mojang, has stated that Toby is a great composer in reply to someone saying that they should get Toby as a guest composer for Minecraft.

In the demo for the Garten of Banban spinoff game "Banban Isle Rangers", there is an Undertale reference.

In a new video by the Youtuber CiblesGD, she counted every tree in Undertale. With this newfound knowledge we can now say there are 9,252 trees in the entirety of Undertale.

During Chapter 4 when you have to make a drink for Alphys, every drink combination has a different name. Alphy's special combination is not mentioned by name in game, but in the files it is called "Dragon's Scar".

There is a typo where Susie says another him in the third textbox of this scene.

Toby Fox will be a composer for Hiveswap Act 3, despite previously stating he wouldn't be as he did not have time.

On James Roach’s resume website, it says that he has been a sound and music consultant for Deltarune from 2018 to the present, even though his name does not appear in the credits.

The swamp area in Deltarune Chapter 5 reuses the same tileset from Chapter 1 for the grass and bricks.

During the development of Deltarune, if there was ever confusion on how a certain pose or animation would be portrayed or done, Toby would act out the animation to demonstrate exactly how he pictured it.

In the Second Sanctuary, leaving Gerson's study after entering it will spawn a battle with Wicabel and Organikk. However this battle doesn't have its own battle message defined, so instead uses the default one meant for test enemies which is "It is known."

There is an unused Key Item in Chapter 3 called the Backstage Pass. As seen by an old preview of the Green Room and by Toby's own admission, originally there was no S-Rank room, which likely means this item would have been used to go backstage and play Mantle.

During the climbing scene in Chapter 4, one frame of Jackenstein's climbing animation has a missing pixel.

In the code for setting up which character uses which talk sound in the boards of Chapter 3, Tenna's talk sound is set to snd_txtspam2, the talk sound for Spamton NEO. But it's never heard in game as Tenna has special code for playing his talk sounds that overrides this variable.

Chapter 3 of Deltarune (Left) has 30 invincibility frames as opposed to the normal 40 in any other Chapter, meaning between hits there is 10 less frames (about 0.3 seconds) of leniency before you can be hit again.