Mario vs. Donkey Kong is a Game Boy Advance spiritual sequel to the first Donkey Kong game for Game Boy, developed by NTS in 2004. The game concept revolves around a combination of platform and puzzle elements, challenging Mario to find keys, reach a locked door, and rescue mini-Marios. [Info from Wikipedia]
Upaluppa and Kiiro found many unused stuff and beta levels still hidden in the final game’s code, as a a prototype version of the MvsDK e-card levels, Mini Marios trapped inside crystal balls as keys, some unused coins / stars and a working level editor. It’s possible that the Level Editor evolved from the unreleased Donkey Kong Plus.
Also, an unused Bomb-Ombs behavior. As wrote by upaluppa:
they start running around scared, just like Shy-Guys do, when Mario is equipped with the hammer! You’ll never see them running normally, because none of the levels with Bob-Ombs includes hammers…
Thanks to Hiccup for the contributions!