I'm sure most of you know that Pokemon Puzzle Challenge is one of many rebranded versions of an original JP game called Panel de Pon, but did you know the game has a secret cheat code that lets you play a version of the game using the original unused Panel de Pon GB assets?
To access this mode on 3DS VC (or an original cart played on GBC), press
Up (2x), Right (4x), Down, Left (10x), Up (4x), Right, Down (6x), B on the title screen.
If performed correctly, it'll take you to a "This game can only be played on GBC" splash screen, like the one you'd see if you put the cart in an OG GB; when you reach this screen, press
A (24x), B (24x) and the game should launch in Panepon GB mode.
Here's what it looks like as seen via Super Game Boy:
https://www.youtube.com/watch?v=8B8YyqyhDoE
The ROM has a lot more unused Panepon data, too--lots of animated character portraits, splash screens, etc. It seems like they were pretty far ahead before the Pokemon rebranding.