Paintings such as this one from the 17th century, in St Paul's Church in Sandomierz, Poland, depict legends of Jews attacking Christians (blood libel). They were thought by some to depict realities, and formed part of the backdrop to anti-Semitic violence.
Today, the marketing of fictional information has become much more sophisticated. In the New Yorker, Eric Randall details a case in which entirely fictional information about a South American animal became encyclopedia fact, hence accepted and replicated:

This gets at an unsettling phenomenon that Stephen Colbert once dubbed Wikiality: the idea that “any user can change any entry, and if enough users agree with them it becomes true.” No matter that Jimmy Wales says he came into this world on August 8th. The consensus says he is wrong. Colbert played with this idea by declaring that Warren Harding was a “secret Negro President.” As proof, he cited an altered version of Harding’s Wikipedia entry. (Colbert failed, in this case, to create enough of a consensus for the change to remain on Wikipedia.) Wikipedia is an experiment in crowdsourcing as much human knowledge as possible, and the logical outcome of that process is that the wisdom of the crowd often rules—as insensible as the crowd can be.

Jimmy Wales’s battle for his birthday and the mischievous edits of Stephen Colbert are amusing, but probably harmless. They have been aired for the public to see. And, in recent years, Wikipedia has made it more difficult to insert unsubstantiated facts into entries. The example of the coati’s new nickname is more insidious, though, because it points to the longstanding existence of errors so minor, obscure, or inconsequential that no one notices and, eventually, they adopt the veneer of truth. Just how many dull facts in this world originated because someone birthed them on Wikipedia? Disproving the idea that coatis are known as “Brazilian aardvarks” might be impossible at this point, not because Wikipedia’s rules make it difficult to cite “a lack of references to ‘Brazilian aardvarks’ in published materials before July 2008” as a source but because it is not technically false. On the Internet, at least, coatis are, in fact, occasionally known as Brazilian aardvarks, and there are numerous references to prove it. More.

Actually, planting that type of information is probably not harmless.
Once enough canards are in circulation, entire fictional scenarios can be created that are difficult to confute because they appear to be well-sourced via constant repetition. Fictional histories far more sophisticated than the painting depicted above could discredit a candidate for public office or libel a group of people. Like photoshopping—it was always there in principle, but never so easy. If the Wikipedians themselves don’t take this more seriously, the rest of us had better.
My own practice is never to use Wikipedia as a source for factual information without checking it with another, non-crowd source. Here is a story about some other questionable Wikipedia entries.

  Denyse O’Leary is a Canadian journalist, author, and blogger.