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Inside the game's files is an unused song called "Mumbo's Rain Dance.".
Contributed by GamerBen144
There's an unused underwater variation of the Freezeezy Peak theme which cannot be heard in-game as the water in that level is too cold (and too shallow) to swim in. Even if the player travels through the icy water submerged after having been turned into a walrus by Mumbo, the music still won't change.
Contributed by CuriousUserX90
Before Rare decided to use Banjo to replace the character Edison (the main character of the canceled Project Dream project that would later become Banjo-Kazooie), they wanted the main character to be a rabbit.
Contributed by VGSavvy
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An early idea for the game was to replace a majority of the dialogue in the game with "Bubblespeak". Thought bubbles would appear above the character's heads featuring an image inside, either still or moving, conveying any advice, warning or info that they're trying to tell. This was to make the dialogue in the game more uni-lingual and funnier.
Contributed by KnowledgeBase
In an early development version of Mad Monster Mansion, there was a painting in the dining room of the windmill from "Twelve Tales: Conker 64" - The game that would later become Conker's Bad Fur Day.
Contributed by Boyobmas
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Inside the game's data is an unused sign that says "On Vacation" It was most likely originally intended to be used for Mumbo's hut in the Winter area of Click Clock Wood. Instead, a Beehive tells Banjo that Mumbo is on vacation.
Contributed by GamerBen144
The original sound for collecting an Egg changed in development. It's not know why it was scrapped, but it might have been because the sound became rather annoying when collecting large amounts of eggs.
Contributed by Carlos
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There is a unused test map in the game's data. It consists of 8 slopes arranged in a circle formation. This test map was most likely made to test Banjo's climbing ability.
Contributed by Carlos
The music for Gruntilda's Lair strongly resembles an old song called "Teddy Bear's Picnic". The composer, Grant Kirkhope, has acknowledged that he probably was thinking of Teddy Bear's Picnic when he wrote Gruntilda's Lair, but doesn't remember it being a conscious decision.
Contributed by Boyobmas
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Inside Banjo's house is a painting of Banjo & Kazooie walking through a forest. This is an image of a world that was originally intended to appear in the game; Fungus Forest. The level wasn't completed in time, so it was put into Donkey Kong 64.
Contributed by Mr. Jellysquid
In an early E3 video, it shows that Banjo and Kazooie could fly at any time they wanted without the use of a flight pad. This was most likely removed to limit the amount of area the player could cover at one time.
Contributed by gamemaster1991
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A poster featuring an early design of Berri from Conker's Twelve Tales can be found in Rusty Bucket Bay.
Contributed by cross307
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An image of Donkey Kong appeared in an early build, but was replaced with an image of Banjo's sister, Tooty.
Contributed by DidYouKnowGaming
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NSFW - This trivia is considered "Not Safe for Work" - Click to Reveal
Grant Kirkhope, the composer of the game, provided the voice of the pots in Mad Monster Mansion that say "Thank You" when the player deposits an egg in them. According to Kirkhope, he had to redo the line several times because Nintendo thought he was saying "Fuck You".
Contributed by KnowledgeBase
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There's an unused firefly sprite which appears similar to the Beehive's Bee sprite which even uses the same wings. It's possible this was intended to be used for the world Mad Monster Mansion, the only level set during night.
Contributed by KnowledgeBase
If you input too many "illegal" cheats in the sandcastle in Treasure Trove Cove, Gruntilda will permanently delete your save file. This won't come into effect until you have saved and quit or turned off the console though.
Contributed by WoollyRex
When you collect a Mumbo Token, a sound effect of Mumbo saying "Ikkum Bokum" plays. This phrase (and most of Mumbo's speech aside from "Oominaka" and a few other sound effects) are all made up of randomly-arranged soundbites. When these soundbites are arranged in the correct order, they say "Come and have a go, if you think you're hard enough."
Contributed by Vipershark
When Mumbo turns you into a creature he says "Oominaka." Grant Kirkhope, Mumbo's voice actor, revealed that at the time of recording the voices for the game his left testicle was swollen. "Knackers" is British slang for testicles, so he thought it would be funny if Mumbo said "Ooh, my Knackers." Thus, every time that Mumbo uses his magic to turn you into a creature, he's actually complaining that his testicles are in pain.
Contributed by SnWBl
The music played inside the Ant Hill of Mumbo's Mountain is a slowed down version of what was originally intended to be the world's main theme. The original theme was partially inspired by the Baby Elephant Walk tune from the film "Hatari!".

The theme that was used for the world Treasure Trove Cove was also the second version that the game's composer, Grant Kirkhope, had written due to the change of the game from "Project Dream" to "Banjo-Kazooie".
Contributed by KnowledgeBase
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Napper the Ghost and Clanker's voices are both Banjo's voice but slowed down and lowered respectively. This was done due to not enough memory space on the cartridge to add additional voices.
Contributed by KnowledgeBase
The idea behind "Stop 'N' Swop" was to do certain challenges/objectives in "Banjo-Tooie," then stop and swap the cartridges while the system was still on. This would have unlocked special items in "Banjo-Kazooie." You can than swap the games again to get the items in "Tooie." This idea was scrapped when the window of time that players could "hot swap" games (the ability to switch games when the power was on) was shortened from 10 seconds to only 1 second. There are still cheat codes that will unlock the special items in Banjo-Kazooie.
Contributed by gamemaster1991
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On the third file select, you can see Banjo and Kazooie playing a Game Boy. The sound effects coming from the Game Boy are from Donkey Kong Land.
Contributed by Bean101
On the title screen, there is a slight chance that you may get an alternate file select animation. These include Banjo being flipped out of his bed and out his window, the wall in Banjo's kitchen spinning around, and Banjo being launched out of his chair.
Contributed by gamemaster1991
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The xylophone that Mumbo plays in the intro is the same xylophone that appears in Candy Kong's Music Shop in Donkey Kong 64.
Contributed by Culex_DarkKnight
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Banjo-Kazooie started development on the SNES and was titled "Project Dream". The game was going to be an RPG, and the main character would have been a young man named Edison who had caused trouble with a group of pirates lead by a man named Captain Blackeye.
Contributed by Culex_DarkKnight