Usually, you think of scams being restricted to "street level" (back alleys, phone conversations and such).
An international gold bar fraud was discussed on CNBC in 2009, but it didn't really seem to do much "word of mouth" travelling (did the viewers actually keep it secret, so much for social communication). Gold bars filled with tungsten were shipped all over the world and also to Fort Knox (important news????...then why didn't your broker or other investors tell you?...this is the first I've heard of it).
Silver has scams attached to it also, but gold is on a higher level.
Even though this is in German, the captions are worth reading. There is alot on youtube.