Pokemon
Mashing B+up would turn any Pokeball into a master ball. Some people still say it works, but I was never able to do it.. I call bullshit..
I remember another one: Mew.
Jesus, whoever started the rumor where you have to get all eight badges, like, 50,000 times, use SWIM, then swallow the game and dump the game back in the system on the fifth full moon of the year to make Mew appear... was a jerk.
But, damn, half my friends tried that and the many different variations. If they would have just waited a while till they could d/l him...