Hmm, I heard a really stupid rumour about the first time I had ever heard of FFVII (Which was about a week after it came out in europe), If you got a super sweeper it would actually kill the emerald weapon for you!
Another one was that you could get Ark summon materia by finding bits of gold throughout the game and combining them in the temple of the ancients.
And finally I heard one that if you killed Ruby weapon you would get a free Gold Chocobo and the means to create one