The Bureau of Engraving and Printing misprints a few of the latest design benjamins. Around 1.1 billion of them.
Not to cast aspersions, but one would think that someone might notice the problem within the first, oh, several hundred million bills.
http://theweek.com/article/index/210...-currency-goof
http://www.dailytech.com/US+Governme...ticle20338.htm