A factoid that is false or unsupported by evidence, but gets into public circulation anyway. Once it is repeated and quoted enough times, it gains a life of its own, and people assume it is true because they get it from multiple sources, even though the original source is flawed or unverified, or the information turns out to be false.
One common fictoid is the idea that people need to drink 8 glasses of water a day to be healthy. There's no sound basis for this recommendation, but it is quoted and given as advice frequently.
Recently (as of new years 2009), one of the big news stories has been the collapse of a fraudulent investment fund run by Bernard Madoff, which turned out to be a ponzi scheme. Although it takes months to go through the records to figure out how much money was involved, an initial estimate was that "up to $50 billion dollars may have been lost". Despite the fact that this was an initial best guess rather than the result of actual auditing, and despite the fact that even a clear definition of "money lost" in this case is vague, this $50 billion estimate has become a fictoid, and is being repeated in the press. Plenty of people believe that it is well accepted that $50 billion is the amount of money that was lost in this fraudulent scheme.