In order to temporarily unlock Mewtwo in vs. mode, you must hold Z, and then press B, Up, L, B, A, Start, A, Up, R. The combination spells out "Bulbasaur".

Hidden inside the code are two unused voice clips. They are presumably from staff issuing a little message to loved ones, as one is in Japanese and reads "Yumi-chan, I love you" and the other reads "I love you Liz."

Pokemon Puzzle League is the only Pokemon game to be released exclusively to the Western market, and not in Japan.