I have an interesting example of that sort of thing.
I was recently looking at the wikipedia page of a band I like, and it contained a claim as to their total worldwide record sales, followed by the legendary [citation needed]. I thought I'd be helpful and add the requested citation -- I was very sure I remembered reading that factoid in an interview with a certain magazine. Said magazine has online archives, so I rapidly found the interview in question, and.... um... no such statement. Oops. Not wanting to give up, I copy/pasted the sentence under debate into google looking for some other source. Result (unsurprisingly) - dozens and dozens of pages which quoted that wikipedia article verbatim. Not only sites which systematically mirror / reuse wikipedia content (last.fm, answers.com, etc) with clear declaration of source & GFDL status -- but also many other sites, some with more of a reputation for credibility and/or original research, such as the BBC.
At this point I started getting a strange "now waaaaaaiiiiiiiit a minute" feeling, so I plunged into the history of the wikipedia article, diffing versions until I found the original addition of the sentence in question. Guess what? I wrote it.
I was rather amused to see this factoid which, although I'd written it with honest intentions and a genuine belief I'd read it in a credible source, apparently turned out to be made up out of my stoned imagination, quoted so consistently across the web by sources WP itself would rate as "notable".
What would stop me from adding that BBC regurgitation (for example) as the requested citation? Well, my conscience. But (excepting the fact I relate this story now) I'm the only person who would know the story of how that "fact" came to be, so what stops someone else from adding that citation with a clear conscience? Nothing.
Of course, this story is not without precedent, but to have it happen with my own words drove the point home somewhat.