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 is a superscript notation used in Wikipedia articles to identify questionable claims without any basis on reliable sources. Outside of online communities, the tag has been also used in real life to poke fun at public / corporate advertisements with dubious messages.
“Citation Needed” was originally introduced as part of Wikipedia’s fact-checking policy in February 2006. Since being adopted into usage, this policy has been strictly applied to all material in Wikipedia articles, lists, and stubs without exception, and in particular to information about living persons and image uploads. Here’s an example of “citation needed” tag as shown in Wikipedia’s article on internet memes:
In July 2007, the popular webcomic xkcd published a comic called Wikipedian Protestor,, which showed a political protester holding a “citation needed” sign at a public speech, probably in reference to President Bush’s 2007 Fourth of July speech. This illustration is the earliest known instance of using the Wiki meta-tag outside of its original context:
One day an idea struck -- what statements are more dubious or outright ridiculous than those in advertisements? Thus, an OM (Operation Mindf*ck) project was born. I had 250 8×2 inch stickers printed, which I handed out to friends, who circulated them further. In true Wiki fashion, the final placement of the stickers is a collaborative effort, now distributed and anonymous. If anyone sees one somewhere, please make a photo! I’ve been tagging my photoset on Flickr with citationneeded and wikiffiti.
This subversive practice of using “citation needed” tags in public soon became known as “Wikiffiti” and was subsequently covered by the news media both online and offline, from BoingBoing and MAKEzine to Boston Globe and Reason Magazine.
Due to the widespread nature of the phrase in various contexts, it is somewhat difficult to measure the extent of its usage outside of the Wiki community. However, a customized Google search of “citation needed” without the term “wikipedia” results in over 640,000 pages within the past year.
Usage in Discussion
Since gaining its catchphrase status, “citation needed” has been used in online discussion forums to humorously point out biased or baseless statements made by others. The xkcd forum thread that resulted from the 2007 comic strip provides early examples of the  within forums, e.g., from one response:
Just think of how this could apply to memes?
So I herd u leik Mudkips? 
I shot a man in Reno. 
ONE CAN NOT SIMPLY WALK INTO MORDOR.