Mm, I got sucked in by the holding-down-buttons-while-throwing-Pokéballs rumors... the version I heard said that you had to hold A and B while you were throwing the ball, let go of B when the Pokémon got sucked into the ball, and then let go of A after the ball had landed on the ground and but before it started wiggling. I still do that, even though I know it doesn't work...
There was one rumor I believed for the longest time, because I wasn't involved in the online Pokémon fanbase and had no way of knowing it was untrue. A friend told me in fifth grade that she had a shiny Ditto (and that much was true) and that, because it was shiny, it could breed with any Pokémon--even the unbreedable ones, the legendaries. She said that if I traded my Mewtwo to her, she would breed it and I could hatch a level 5 Mewtwo, which seemed like a good deal to me. This was in fifth grade and I believed her, but I never remembered to bring my game to school so we didn't trade. Now I'm wondering if she just hadn't tried to breed any legendaries and simply assumed that because Ditto could breed with normal Pokémon, it could breed with all of them, or if she was trying to trick me out of my legendaries.