These glitches are FAKE so don't try them out

Breed an alakazam and a meganium and you'll receive an egg with celebi inside

Train a squirtle to lv.100 without evolving him and he'll lay an egg with togepi inside

breed a cyndaquil and a sandshrew and out of the egg you'll get a sandshrew with flaming back

If you spell entei with unown in your party and keep an open spot entei will appear in the open spot

On fire red and leaf green you can train your pokémons to lv 150 and they will have EX where there level would be. (lol, I can't believe that I actually heard this somewhere)

If you go to the space center, and it's the 50th rocket launch, talk to everyone, a jirachi should appear somehow, same for deoxys and 100 rocket launches, I also heard this for 999, it's absolutely wrong, the rocket launches are (I think) just a way to count the number of weeks that have passed since you started the game.