1.The most common rumor is that of finding MEW under that truck near the S.S. Anne. You can't even move it with Strength. The only way to get MEW in the American Version is with the Game Shark, the code being 0115d8cf to find him in the wild. The truck is just a Japanese prank!
2.Have you heard about that code that you have to beat the Elite 4 30 times in a row and then you'll go to a secret city? Well, IT'S NOT TRUE! Don't waste your time!
3.Hmm...I heard something about a stone called the Mist Stone. It is MOST LIKELY that it is just a lie.
4.Close your eyes if you must, I'd hate to admit it, but those codes like you having to run from a Haunter, then Gastly, and then Cubone to find Pikablu are LIES. OH NO! Also, there is another one in Viridian Forest where you find a mix Pokemon "Charticuno" (Charizard+Articuno). Also a lie. Besides, fire and ice DON'T MIX!
More rumors to come soon!