Pokémon Puzzle Challenge

Panel de Pon

If you load Pokemon Puzzle Challenge on the original Game Boy, and then press the A and B buttons 24 times each (A first, B second), you can access what appears to be an unfinished prototype of "Panel De Pon GB". It also seems like there were initially plans to keep the fairies intact in the European and American releases of "Panel De Pon GB", as opposed to replacing them in a similar vein to the SNES game.