Kirby’s Adventure is a platform game developed by HAL Laboratory and published by Nintendo for the NES in 1992 (Japan) and 1993 (USA andEurope). As we can read at Wiki Rusted Logic, the game features a great amount of debug rooms and unused content, some of which would end up in future Kirby games.
Miniboss test rooms. Not just any minibosses though — these rooms have the faster, more difficult versions of Bugzzy, Poppy Bros. Sr., Rolling Turtle, and Mr. Tick Tock that are found in stage 7-2’s hidden route.
Rooms 0142-0144: These rooms, however, weren’t used. Meta Knight battles for the layouts of rooms #142 & #144 still exist in the game, but not for room #143.
Room 0145: A pretty cool cannon puzzle that’s only accessible from the debug room. To get the cannon to fire, you have to pound in the stake with the Hammer or Stone abilities, then quickly inhale the Laser Ball to get the Laser ability. Next, quickly fire a laser at the slope at the bottom before the ice block appears to block you, then hop into the cannon. It’s tricky to get it timed right, but it does work.
Unused Ability : these tiles of a tiny Kirby are found in sprite bank 8C, along with the graphics for the Mike and Ball abilities. These graphics were possibly used for an ability that no longer exists in the game. The oddly sized Kirby would have been used for a shrinking animation […] This Mini ability might have been replaced with the UFO power [..] 12 years later, this ability would finally be implemented in Kirby and the Amazing Mirror. However, in that game, you can’t slide while using this power.
Also, CrashAndSonicChao found the lost Mini Kirby sprites thanks to a sprite editor:
Thanks to ORKAL‘s Youtube Channel we can see an interesing video that shows many of the unused content.
You can find more about the Kirby series in the WiKirby!