Super Mario Odyssey
Super Mario Odyssey
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In the game's E3 2017 demo, the rooftop pool in New Donk City was originally deep enough to completely submerge Mario; however, it is much shallower in the final game, with the water only going up to Mario's neck. It's likely that this was changed because the original depth presented the risk of Mario drowning.
Contributed by game4brains
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The game's official art book, The Art of Super Mario Odyssey, includes concept sketches for a never-realized scenario where Bowser uses his own version of the Capture ability to possess Princess Peach, who would adopt Bowser's physical characteristics.

The concept art attracted attention online following the book's publication, as it coincidentally predated the creation of Bowsette, a viral fanmade design for Bowser wearing the Super Crown power-up from the Nintendo Switch port of New Super Mario Bros. U. Like the concept art, the Bowsette design mixes together attributes from both Peach and Bowser. The original comic depicting Bowsette was itself a parody of Super Mario Odyssey's ending, Spoiler:depicting Mario going out on a date with the Super Crown-wearing Bowser following Peach's rejection of the two.
Contributed by game4brains
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The art of Super Mario Odyssey reveals that Rosalina was going to appear in the game as a non-playable-character in casual attire with a guitar. The book also reveals several unused outfits for Mario, such as the green Sprixie Princess from Super Mario 3D world, a Toadette costume, a costume of a Luma from Super Mario Galaxy, and a costume of Mario in a Japanese school girl outfit.
Contributed by CLXcool
Stacking 30 goombas is the normal limit in game, but it's possible to stack 200. Going over this amount crashes the game.
Contributed by Kakariko Kid
Actual cooking utensils were used to make the Luncheon Kingdom soundtrack.
Contributed by ChocoPain
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Early boxart of the game features Mario in his sombrero outfit from the Sand Kingdom in the bottom left corner instead of his snorkling outfit from the Seaside Kingdom. This was likely changed in response to a backlash about the outfit, to help avoid offending any potential customers.
Contributed by CuriousUserX90
According to producer Yoshiaki Kazumi, in an interview with Polygon, the reason why Pauline made a comeback in this game was because they wanted to give players something familiar to focus on in a strange place.

“When we were deciding which Kingdoms to have in the game, we knew we wanted the city environment, and we wanted something that would have resonance with players. We knew that players know Pauline so we of course wanted to make her the mayor of New Donk City. It's important to note that though they are called Kingdoms, they do not necessarily have royalty, and that's why she's the mayor.”
Contributed by GamerBen144
Talking to an NPC counts as jumping according to the in-game jump counter.
Contributed by ChocoPain
According to producer Yoshiaki Koizumi. Pauline was originally conceived as the princess of New Donk City, but the idea was later scrapped and she became the mayor of New Donk City instead.

"In the past we used to have a game taking place in a city, and Mario also appeared in it(which pointed towards Pauline). At first they wanted to make her the princess of New Donk City, but the world setting of that city doesn’t have princesses. That’s why she became the mayor instead.”
Contributed by CLXcool
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According to Yoshiaki Koizumi, the song 'Jump up, Super Star!', came from developing Pauline as a character.

"As we were developing Pauline more as a character, we know that she was going to be interested in jazz. It was interesting for us to have the first song in a Mario game with vocals. Have you guys been finding yourself singing it?"
Contributed by CLXcool
During the "Jump Up, Super Star!" song sung by Pauline, the mayor of New Donk City, the 25m theme from Donkey Kong can be heard when she sings "Odyssey, ya see". Also whilst she sings, her dance mimics her sprite movements in Donkey Kong.
Contributed by CuriousUserX90
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Inside the map that Hint Toad is seen holding, if you're able to look close at it you can make out the layout of the level Bob-omb Battlefield from Super Mario 64.
Contributed by KnowledgeBase