If you've visited Wikipedia in the last five years, you might have heard about the "Bicholim Conflict of 1640-1641 [which] was a period of armed conflict between the Portuguese rulers of Goa and the Maratha Empire led by Shivaji Bhonsle in the northern regions of Goa [India]".
Except, none of that ever happened: the "Bicholim Conflict" was completely fake, a hoax created by an unknown user.
And, it took Wikipedia over five years to realize that and delete the article. (An auto-generated book persists, link).
Before that time, it was nominated for a "Featured Article", possibly by the hoaxer. While it wasn't approved for that, it did receive a "Good Article" award from Wikipedia.
Here's the Wikipedia Bicholim Conflict page as of October, 2010.
What other small and large hoaxes persist in Wikipedia? Unless you're already an expert in a particular field, you'll never know. Every single "fact" printed in Wikipedia has to be double-checked with a credible source because you'll otherwise never know if that someone's playing a hoax.