There's a few unused messages hidden in the ROM that differ from the US and EU version. In the US, there are 3 messages of "Hello there!" whereas in the EU version they read: "Wow Salem, so this is what the inside of a hex editor looks like..." "Hello there!" "Hey look! Someone ripped our game!!!"
There is also a large chunk of uncompiled code, and for some reason the file path to the developer's ZoneAlarm settings: "C:\PROGRAM FILES\ZONE LABS\ZONEALARM\ZONEALARM.E"
When the Japanese version's passwords are put together in a string, they make up the first 114 digits of pi. (3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798214808651328)
The PAL and NTSC versions of the game use vowels instead of numbers, and are mostly made of random strings.
Although Chapter 1 was given a surprise release in 2018, with Toby Fox giving no prior public indications of its existence, Deltarune had been teased in secret since at least 2015. Three months after Undertale released, the fan site DeltaRune.com announced that it would be rebranding to Dreemurr.com at Fox's request; Fox didn't tell the owners of Dreemurr.com why he didn't want them to use the domain. However, after the fan site fulfilled his request, Fox used deltarune.com to host an image called him.png, a passage of Wingdings text only readable by turning up the brightness using image editing software.
While the text initially parroted the phrase "THIS NEXT (space) EXPERIMENT (space) SEEMS (space) VERY (space) VERY (space) INTERESTING" from the unused room_gaster event in Undertale, by July 1, 2016, the text was edited to read "THREE HEROES APPEARED (space) AT WORLD'S EDGE", later being edited again by August 17 to say "THREE HEROES APPEARED (space) TO BANISH THE ANGELS HEAVEN". Both revisions quote portions of the legend that Ralsei recounts in Chapter 1, indicating that these parts of the game's backstory were already conceived by this point.
Following the release of Chapter 1, deltarune.com would be refurbished as the official website for Deltarune itself. Consequently, earlier versions of the site only survive through snapshots on the Wayback Machine, a URL archiving platform hosted by the Internet Archive.
At the very beginning of Appendix v. The Lost Falls, after Ida presses the first switch in the level to raise the pillar the Totem is on up to where she is, a set of symbols carved into two sides of the pillar are revealed. These symbols are part of the ancient Zuish language from the game Fez, but they are used incorrectly in order to fit the message on the pillar (i.e. vertical spelling, no use of spaces, line breaks in the middle of words). When translated, it spells out this message:
TOSR ONDO GEAU OINS ATG LIE
"TO GO ALONE IT IS DANGEROUS"
This not only acts as a reference to the popular quote at the start of The Legend of Zelda to show Ida receiving help, but also acts as foreshadowing to the end of the level where Spoiler:the Totem is destroyed by a crushing stone pillar while helping Ida get past it, forcing her to go through the rest of the Forgotten Shores levels on her own.
The background music for the "Merch Cemetery" stage has the filename "97 115 99 105 105 10", which when converted into ASCII spells "asciiLF". It is not known what this title means.
In Virtua Fighter 3tb, Jacky and Kage both have a secret win pose when holding Down, Guard, Punch, and Evade. They will perform "V for Victory" poses. If Kage is in his 2P costume and gets an "Excellent" victory on Akira's stage, he will perform a secret win pose where a metal pan will fall on his head.
After defeating Solidus Snake at the end of the game, he falls to his death off of the Federal Hall building. Just before he falls however, if you look closely, you can see his lips move slightly in the brief moments that they are not covered by the collar of his exoskeleton. By replacing Solidus' model with Solid Snake's (who shares most of Solidus' facial features) through mods, it reveals that Solidus was actually saying one last message to Raiden. However, the dialogue during this moment is still muted, so Solidus' last words remain unknown.
According to Raww Le Klueze, global admin of the Suikosource forums, there is a hidden marriage proposal spoken by The Castle Keeper zombie in Neclord's castle that can be found in the game's files in ve3.bin and ve6.bin. There is no known way to access it in-game. Translated to English, it reads:
"Kazuko I really love Kazuko Even after 10 years Even after 20 years Even after 50 years I will always, always love her. ......... Teeheehee! Thank you for always being there. And for always supporting me! PS Let's get married! Mwah!"
It's unknown who Kazuko is or who wrote this proposal. This text is only believed to be present in the Japanese version of the game, with the North American version replacing it with a repeat of some past dialogue with the zombie before ending with the "Mwah!" replaced with a single "Yuk.".
According to Raww Le Klueze, global admin of the Suikosource forums, there is a secret 10th Old Book. There is no known button combination to access it, and acts more like a hidden programmer message with only one page. Translated to English, it reads:
"Nomo Hideo"
Congratulations on starting the Major League Baseball All-Stars Game!"
Hideo Nomo was a Japanese baseball player and a starter in the 1995 Major League All-Stars game in the United States - the same year Suikoden was released in Japan - and is known as the first Japanese citizen to join an American major league team after having played professionally in Japanese major leagues.
After recruiting Hugo, you can give him your Old Books and read them in the Castle's library on the third floor. In the Japanese version of the game, while reading these books, dataminers years ago found that a seemingly unused crudely-drawn sketch of the game's cover artwork was loaded into the VRAM. In reality, this sketch was not unused, but was actually part of an obscure Easter egg: a secret 9th Old Book. While highlighting the 8th Old Book titled "The Kalekka Incident", press Square, Circle, Triangle, and Cross at the same time. This will unlock the hidden 9th volume entitled "Hirotaka Development Diary" (presumably referring to the game's producer Hirotaka Ishikawa). A very rough English translation of the book's first four pages can be seen below:
- Pages 1 & 2 - (About the library)
"The Game Master used a ”Please/Favor Card”.
Thus my daily assistance began.
July 10th (Mon)
Made the BG graphics.
July 11th (Tue)
Made the page graphics.
July 12th (Wed)
Entered the text.
- Pages 3 & 4 - (About the mural)
The Game Master gave me the next job.
July 13th (Thu)
Made the images for the pieces.
Didn't have time so had to finish in one day.
(About the Stone Monument)
July 17th (Mon) – July 21st (Fri)
It was difficult because it doesn't have a face.
(About the Bath)
July 24th (Mon) – July 29th (Sat)
I want to have a bath too!"
The last two pages of the developer diary featured the crude sketch of the game's cover art alongside a request to mail in your opinions on the game to a listed address that is presumably somewhere in Kanda-Jinbōchō in Tokyo (Note: this full page has not been translated). It's not known if this secret is in the Sega Saturn version of the game, but it is confirmed to work on the PlayStation Portable port, Genso Suikoden I & II. However, this port has the last two pages (including the sketch of the cover art and request for opinions) removed, reducing it to four pages instead of six.
Courage the Cowardly Dog: Creep TV has secret codes and maps that went undiscovered until 20 years after its release due to a glitch making them inaccessible. The codes include the "daScore" code, which shows a counter of how many ghosts you've zapped, and the "mirage" code, which expands the game map infinitely and allows access to the secret maps. On one of these maps, you can acquire a pair of glasses as an unused item, and another map containing an oasis in the Middle of Nowhere that also has a sign simply reading Eustace's catchphrase, "STUPID DOG!". However, if you use the glasses to read the sign, it will instead say "Wee...amazing! You found it! GOOD DOG!"
Within the game's code is a commented-out test string transcribing the Tragedy of Darth Plagueis the Wise, a monologue by Chancellor Sheev Palpatine from the 2005 film Star Wars: Episode III – Revenge of the Sith; the monologue previously became a popular copypasta in the late 2010s, midway through the game's development. By nature, this text cannot be seen in the normal course of play and is only visible through observing the game's data.
In Slayers X, the Boise Potato Festival contains a crevasse leading to a bottomless pit. If the player manages to land on an outcropping just below the kill plane, they can access a hidden tunnel littered with messages by Adam Chase, a character in the Hypnospace Outlaw universe who assisted Zane Lofton with developing the game. According to Chase, he helped make the game as part of a promise he made with Lofton back in high school and that even though he now despises Lofton, he pities his situation as "a deadbeat dollar store manager" who believes that the game will help him and his son escape their current lot in life. The end of the hallway contains a photo of Lofton in his underwear peppered with "BWL" (short for "bursting with laughter"), the Hypnospace Outlaw equivalent of "LOL."
The Acorn Archimedes version of the game contains an additional hidden message in the list of software interrupts for the TeqMod module (identical to the one found in the game Manchester United Europe):
A secret message can be found on the title screen of the NES version of the game with the use of two controllers. At the title screen, select "CONSTRUCTION" on the menu. Press Start on Controller 1 to enter the mode, then press it again to return back to the title screen. Repeat this seven times. After the seventh enter-and-exit, hold Down on Controller 1 and press A eight times on Controller 2. Then, hold Right on Controller 1 and press B twelve times on Controller 2. Finally, press Start on Controller 1 to activate the message featuring an animation of a green droplet falling.
"THIS PROGRAM WAS WRITTEN BY OPEN-REACH WHO LOVES NORIKO ....."
The most likely person as to the identity of "Open-Reach" is Tomcat System's founder and programmer Ryoichi Ookubo because of the similarities between "Reach" and "Ryoichi". It should be also noted that his name is hidden twice in the game's ROM, and from one fact pointed out by a Russian LiveJournal user, that the name Open-Reach appears as "OPR" on the title screen of another game Tomcat System developed, the 1991 Famicom game Castle Quest published by Hudson Soft.
As for Noriko, her identity remains unknown. In the comments section of the sourced video, one commenter claimed that Noriko was Ookubo's daughter who was three years old at the time of the game's development, while the aforementioned LiveJournal user claimed that Noriko was a romantic partner, claiming her to be a former Namco graphic designer named Noriko Ikegawa (whose name would later appear in the credits for the Nintendo game Animal Crossing as a model designer), but there is no known substantial evidence to support either of these claims.
In the King's Mausoleum, there is a very dark room you can go to that has several stone faces hanging from the ceiling, and a stone pedestal in the center of the room that can be set on fire to brighten it (to better hear this Easter egg, you should not light it, and turn off the music). If you stand in front of the pedestal, several distorted voices can be heard throughout the room. Reversing them reveals that they are actually voice clips of the Radio Operators from the English dub of Armored Core 2: Another Age.
If the player enters the following names starting with the word "Krunk" as their profile name, hidden messages will appear:
Krunk: "You heard about the time I killed me a couple of Heroes?" Krunkl: [same message as above] Krunki: "My hands are red with the blood of thousands." Krunko: "You know I once burned down a whole village?" Krunkn: "I once killed a White Balverine, I did. It's true."
Note that the last letter in each variation of "Krunk" spells "lion", in reference to Lionhead Studios.
Although Chapter 1 was given a surprise release in 2018, with Toby Fox giving no prior public indications of its existence, Deltarune had been teased in secret since at least 2015. Three months after Undertale released, the fan site DeltaRune.com announced that it would be rebranding to Dreemurr.com at Fox's request; Fox didn't tell the owners of Dreemurr.com why he didn't want them to use the domain. However, after the fan site fulfilled his request, Fox used deltarune.com to host an image called him.png, a passage of Wingdings text only readable by turning up the brightness using image editing software.
At the very beginning of Appendix v. The Lost Falls, after Ida presses the first switch in the level to raise the pillar the Totem is on up to where she is, a set of symbols carved into two sides of the pillar are revealed. These symbols are part of the ancient Zuish language from the game Fez, but they are used incorrectly in order to fit the message on the pillar (i.e. vertical spelling, no use of spaces, line breaks in the middle of words). When translated, it spells out this message:
The background music for the "Merch Cemetery" stage has the filename "97 115 99 105 105 10", which when converted into ASCII spells "asciiLF". It is not known what this title means.
After recruiting Hugo, you can give him your Old Books and read them in the Castle's library on the third floor. In the Japanese version of the game, while reading these books, dataminers years ago found that a seemingly unused crudely-drawn sketch of the game's cover artwork was loaded into the VRAM. In reality, this sketch was not unused, but was actually part of an obscure Easter egg: a secret 9th Old Book. While highlighting the 8th Old Book titled "The Kalekka Incident", press Square, Circle, Triangle, and Cross at the same time. This will unlock the hidden 9th volume entitled "Hirotaka Development Diary" (presumably referring to the game's producer Hirotaka Ishikawa). A very rough English translation of the book's first four pages can be seen below:
In Slayers X, the Boise Potato Festival contains a crevasse leading to a bottomless pit. If the player manages to land on an outcropping just below the kill plane, they can access a hidden tunnel littered with messages by Adam Chase, a character in the Hypnospace Outlaw universe who assisted Zane Lofton with developing the game. According to Chase, he helped make the game as part of a promise he made with Lofton back in high school and that even though he now despises Lofton, he pities his situation as "a deadbeat dollar store manager" who believes that the game will help him and his son escape their current lot in life. The end of the hallway contains a photo of Lofton in his underwear peppered with "BWL" (short for "bursting with laughter"), the Hypnospace Outlaw equivalent of "LOL."
A secret message can be found on the title screen of the NES version of the game with the use of two controllers. At the title screen, select "CONSTRUCTION" on the menu. Press Start on Controller 1 to enter the mode, then press it again to return back to the title screen. Repeat this seven times. After the seventh enter-and-exit, hold Down on Controller 1 and press A eight times on Controller 2. Then, hold Right on Controller 1 and press B twelve times on Controller 2. Finally, press Start on Controller 1 to activate the message featuring an animation of a green droplet falling. 
