Burger King's account was hacked to look like McDonald's. Then there was Jeep's account, which sent out plenty of nonbrand-friendly language. And finally, and more seriously, the the Associated Press' account was taken over by hackers who told the media outlet's millions of followers that bombs had exploded at the White House.
http://abcnews.go.com/Technology/twitter-beefs-security-factor-verification/story?id=19235783#.UZ1mUrUm2So