Debunked: Go to the River level. When you get to the Metapods, throw an apple at them all, then look up very quickly. You should see a tail from a Mewtwo. Take a picture of it. When you get Professor Oak’s rating, he says it is not worth any points.
My explanation to the sources story is that when you throw apples at the Metapods, there is a chance that a Poliwag/Magikarp might jump out of the water high enough so that their tail is near the top of the trees. The source may have been confused by another Pokemon. Secondly, there’s no reason for Mewtwo to appear in this fashion, let alone in a “glitch” form, considering he is not in the game at all, except for in picture form.
by shelly (Source), debunked by Vaughn
In the Cave course, save one of the Jigglypuffs. While it’s still in the air, hit it with a Pester Ball. Now it will show Jigglypuff holding a microphone even though it’s not supposed to!
by Ryan “MetroidMoo” Ammerman (Source) and Vaughn