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In Kirby Star Allies' Soul Melter EX difficulty of The Ultimate Choice, Void's opening attack greatly resembles Yukari Yakumo's Spell Cards "Boundary Between Form and Emptiness" and "Boundary Between Wave and Particle" from Touhou Bunkachou ~ Shoot the Bullet.
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Despite being removed from Fire's moveset after Kirby's Return to Dream Land, Spinning Fire Breath is fully programmed and finished in Kirby Planet Robobot, but goes completely unused. The move can be re-enabled by changing only 1 byte in the game's code.
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Left over in Bandana Waddle Dee's files in Kirby's Return to Dreamland is an unused animation which is not used in the final game.
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Did you know you can actually play Kirby Tilt & Tumble on the Nintendo Gamecube using the Game Boy Player? You have to physically move the console around because the cartridge is responsible for the gyro controls.
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Video showcasing what Kirby's Return to Dreamland levels looked like if you took the camera beyond it's original position.
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In Kirby's return to Dream Land Deluxe, new sound effects were added to many boss attacks.
These are most likely to better telegraph the attacks, and to bring Return to Dream Land up to par with the sound design of later games.