I remember I disproved a lot of rumors in 4th gen.
Like poster above, mythbuser list.
Govto Eterna forest with a Shiny Jirachi, Pikachu, and Mantyke to open a secret room where you can catch Shaymin Lava Form
Ok, I went to every single spot ere, and no secret room whatsoever.
If you evolve Phione into Manaphy, you get a special Water Key to unlock the secret stash in the Exchange Building in the Battle Park.
Basically disproves itself as Phione and Manaphy, as legendary Pokémon, do not evolve nto or from eachother.
If you go to Spear Pillar early, and use the Azure Flute to capture Dialga, Palkia, and Mewthree (this person thought Arcues was the fabled Mewthree) and then go to the Spear Pillar later on, when Cyrus is there, he will start slapping you and have a mental breakdown, and then falling off the mountain,
Dialga/Palkia does not appear if you got to the Spear Pillar early, and Mewthree does not exist.