The official Associated Press feed on Twitter was hacked this afternoon, and those responsible started sending messages of news events that did not occur. For example, a little after 1 p.m. eastern, @AP "reported" that there were two explosions at the White House and President Obama had been hurt -- though this did not happen.
But for a minute or so, folks who rely solely on twitter for news didn't know to ignore the AP's feed, and thought an actual crisis was underway. For example, take a look at what happened on Wall Street. This chart shows today's Dow Jones trading.
I added an arrow, but I think you can probably guess exactly when Wall Street saw the "news," and when traders realized the AP had been hacked.
The morals of the story: (1) Wall Street watches Twitter. and the notion that social media doesn't have a meaningful, real-world impact is an antiquated concept; and (2) password security matters.
Update: At the start of today's White House press briefing, an AP reporter told the rest of the press corps about the "false" reports they should ignore. Jay Carney added, "Good. I thank you for that. I appreciate that. And I can say that the president is fine. I was just with him."