Reuters hacked, fake news posted
zdnet.com
zdnet.com
http://www.nytimes.com/2008/09/09/business/09air.html?_r=1
and imagine it being posted intentionally by someone with a large short position.
On a more serious note is would probably be worthwhile for news agencies to to at least put a digital fingerprint on their published stories to raise the cost of creating a credible fake story.
Some background links:
http://www.newscientist.com/article/dn21506-assad-masses-syr...
http://www.theworld.org/2011/06/syrian-government-takes-the-...