I have heard of the Yoshi trick. You're supposed to play Poke Flute 93 times in front of Mewtwo. I tried this with a Yellow version but screwed up at 33 when I talked to him. But that was several years ago, I was gullible, and now it's safe to say that code is just a load of
*EDIT: hunter160, the Yoshi cheat is supposed to make Yoshi appear as a Pokemon you can fight with. But, even if it really was Yoshi it'd look like Missingno. or something. Whoever told you this cheat is lying, or mistaken because he heard it from someone else, who heard it from...and so on.