Pokémon Pokopia was first conceived during the development of Pokémon Scarlet and Violet by director Shigeru Ohmori, who had been looking for new directions for the series to go in. Thinking back to his work as a map designer for Pokémon Ruby and Sapphire, Ohmori decided to develop a game where the player could create their own Pokémon habitats. Due to the game being a sandbox game, The Pokémon Company recommended Omega Force, a division of Koei Tecmo that had previously worked on similar titles such as Dragon Quest Builders 2 to work on the game.

Heavenly Guardian was originally developed as a new installment in the Pocky & Rocky series, under the name Kiki KaiKai World. However, after Taito was bought out by Square Enix in 2005, Starfish was forced to drop the license due to unspecified issues on Square Enix's end. Rather than scrap the game entirely, the developers retooled it into an original IP, branding it a spiritual successor to the Pocky & Rocky series.

When director Tez Okano first pitched the game to Sega's management, they initially didn't believe he was serious about making the game and thus didn't give him a budget. When he pitched the game a second time, this time clarifying that he genuinely wanted to make the game, Sega president Hisao Oguchi agreed to give him a budget. Okano worked on the game in secret for two years before presenting the finalized version, believing that "anything could have happened" if somebody found out about the game's development. Due to Sega's financial status at the time, they authorized the game's release under the belief that it wouldn't hurt the company's image.

According to 3D artist Eric Arroyo, he built a Xenomorph Warrior from the Alien series inside Halo 3's engine as part of early experiments for a potential Aliens style encounter. The idea never advanced beyond prototyping, and plans for a crossover with the Alien franchise were scrapped with it.

According to former game director Isaac Christopherson in a Twitter Q&A following the release of Chapter 5, the Prototype (who makes his full debut in the chapter) went through various changes compared to his original draft for the game. Although the twist that the Prototype was actually Spoiler:Ollie and him having a jester head were both always planned, his design was originally planned to be more akin to Catnap's shrine seen in Chapter 3, as well as having multiple Grabpacks in his back to lift himself. He also had a different personality, with Christopherson noting that he would "never speak as though he knew he was the bad guy". Additionally, while the idea of having Elliot Ludwig be the human origin of the Prototype was considered, they didn't go through with it due to being "too obvious".

When the game was released in the US, the stage order was almost completely rearranged from the original Japanese release, with the exception of the final stage being the Cave in both versions of the game. Perhaps most notably, the Pirate Ship was the second-to-last level in the original Japanese version, but was changed to be the first level of the US version.

According to director Goichi Suda, Romeo's design was based on the main character of a cancelled project called "Zombie Rider", whose main character would have been a human-zombie hybrid. The project never came to fruition, but Suda decided to use the design for Romeo, who is similarly half-dead. These ideas were tweaked in the process, such as updating his backstory so that his grandfather Ben used the Dead Gear to bring him back to life.

Within the game's data is an unused placeholder cover image for a custom song, consisting solely of a stock image of a picture frame and comedic text congratulating the player for importing their own music into the game. The image was likely used to test the PC version's custom beatmap feature, which, incidentally, was not officially announced until nine days after the placeholder's discovery.

Originally, Marge's megaphone had a limit on how many people she could recruit into her mob. In the final version, however, there is no limit, and she can recruit as many as she wants. In the game engine, her original upgrade would increase the number of mob members she could gather, rather than boosting her blowback strength as it does in the final game.

Mewgenics includes the ability to obtain cats with real-life congenital abnormalities, playing off of the stigmatization of them in the actual eugenics movement. According to co-creator Edmund McMillen, he and co-director Tyler Glaiel received many pitches and ideas during development from fans living with these conditions on how to represent them in a positive light in the game. McMillen (who himself has dyslexia) wanted to include them to show that they can carry in-game advantages in spite of their concurrent drawbacks (i.e. cats born with dwarfism have exceptionally high luck), tying these into the game's greater theme of "making it work [with] what you're given". In line with the fan participation, the in-game portrait for autism is modeled off the TBH Creature (also known as the Autism Creature), an MS Paint drawing by Twitter user @acmeiku that went viral on Tumblr and was jokingly referred to by some as an unofficial mascot or spokesperson for the disorder.

The original mechanic for Bart's slingshot was slightly different. Instead of his ammo being tied to his power meter, Bart's default ammo would have been unlimited. Players could also collect limited special ammo throughout levels, which would increase the slingshot's range and damage. Although this system was not used in the New-Gen version, the special ammo mechanic remained in the Old-Gen and DS versions.

In the 2025 Nintendo Switch version of Super Mario Galaxy, at the start of the mission "Tarantox's Tangled Web", Captain Toad's line from the original game was changed from mentioning only a single Toad to multiple Toads, since none of the Toad Brigade went alone to go looking for the Power Star in the said mission.

The title and cover art for "ball is infinite" reference a notorious Easter egg from Sonic CD, where toggling the sound test to a specific combination produces a screen depicting multiple copies of Sonic the Hedgehog with humanlike faces and text atop translating to "Infinite fun. Sega Enterprises. –Mazin Picture". The "ball is infinite" cover art similarly depicts multiple copies of Beat mimicking Sonic's pose with Quaver's face superimposed atop hers. Also like the Sonic CD screen, the text on the "ball is infinite" cover is written in Japanese with "infinite" represented by an infinity symbol, though an English translation is provided below. A looser translation of the Sonic CD screen's text, "Fun is infinite with Sega Enterprises," forms the basis of the song title.

"SQUARE UP", a song contributed to the Breakout Edition expansion by American electronic musician Jamie Paige, prominently references her 2022 song "(:women_wrestling:)" (itself later included in the game's first paid DLC pack). Not only do both tracks utilize fistfight imagery as a metaphor for love, but "SQUARE UP" also features multiple lyrical callbacks to its predecessor. The line "But there's a bullet kept inside my zone" references the "(:women_wrestling:)" line "I keep a bullet with your name inside my zone", and the "SQUARE UP" lines "Back to back / Beat some ass" are directly sampled from the 2022 track. The callbacks to "(:women_wrestling:)" are furthered in the music video for "SQUARE UP", two scenes of which feature illustrations of Gumi wearing the same outfit from the 2022 song's cover art. Both the illustrations and the "(:women_wrestling:)" cover art are additionally done by the same artist, Bearvamps.

Originally, all 100 of the game's endings were going to be directed by co-director Kotaro Uchikoshi, with the writing being split up across the various in-house writers of Too Kyo Games. However, as the game received more content and deadlines approached, fellow co-director Kazutaka Kodaka became involved in various routes of the game. One of these routes was the "Cult of Takumi" route, which Kodaka began working on after learning that nobody else was working on a "damn sexy" route. The original plan for this route was for it to be a "sexy harem narrative", but as he wrote more, the story as seen in the final game became darker and more twisted.

According to Pokémon illustrator Yusuke Ohmura, the Ultra Beasts' designs were created with the aim of making them look as little like traditional Pokémon as possible, making use of "combinations of motifs that could never realistically exist" as well as unused Pokémon designs that Ohmura had submitted but had been rejected by primary Pokémon character designer Ken Sugimori. The reasons why these designs were rejected were used as a basis for the designs of the Ultra Beasts.

There's a first-person view mode only present in the E3 2007 build of the game that allows you to freely look around the environment; this mode can be accessed by pressing the left and right sticks together in the build. In this mode, enemies can still detect and attack you, but you won't take any damage.

Upon reaching Acquaintance level with Honey, she will have a chance to say: "My kids' friends used to call me "Stacy's mom". I've had some weird nicknames over the years, but this one stumped me... My daughter's name isn't Stacy!" This is a reference to the song "Stacy's Mom" by the American rock band Fountains of Wayne.

According to the game's director Hideki Kamiya, the Bangle of Time item and its ability to stop time was inspired by The World, a Stand from the manga series JoJo's Bizarre Adventure.

D-Hopper was originally developed and slated for release in the Summer of 1996 under the name Dragon Hopper. The game centered around Dorin, a young dragon prince who must save his family and love interest after they are all captured by a corrupt prime minister. Despite being completed, the game's release was cancelled due to the critical and commercial failure of the Virtual Boy. It would later see an official release in 2026 for the Nintendo Switch Online service alongside another unreleased Virtual Boy game, Zero Racers. It's believed the game's North American name was changed to D-Hopper because the trademark for the name "Dragon Hopper" had been claimed in 2024 by the online casino game developer Push Gaming.