North Korea makes extremely high-quality counterfeits of US $100 bills, called "supernotes", using the same paper, same type of printing press, and similar ink as the real bills. They got the equipment from the Swiss, who are notorious for enabling financial crime. The notes do have flaws, likely put there on purpose, if one knows where to look, to allow their customers to distinguish them from the real thing. "the makers of the supernotes had likely included their few tiny flaws on purpose, so that they and their customers could tell the difference between the counterfeits and the real thing. Otherwise, there would be nothing to stop criminals from ripping off their suppliers by purchasing supernotes—which typically cost around a third of their nominal value—with other supernotes."
The US knows that the North Koreans do this, and it allows them to, for fear of alienating them.
"The Illicit Activities Initiative had a lot more planned. The final stage, which David Asher says President Bush had been fully briefed about, would have been the unsealing of criminal indictments. “We could have gone after the foreign personal bank accounts of the leadership because we could prove they were kingpins,” Asher says. “We were going to indict the ultimate perpetrators of a global criminal network.”
Instead, the Bush administration suddenly decided not to proceed. What Asher describes as the “ultimate non-aggressive containment strategy” was unexpectedly curtailed. The reason: the administration believed that it risked provoking a permanent North Korean withdrawal from talks on its weapon and missile programs. Hayden says, “Suddenly, the rules had changed. The diplomatic piece of this came swooping in, and the imperative became ‘Let’s get them to the table,’ and that meant everything had to be ratcheted down.”
The fake notes are almost impossible to distinguish from the real things except by FBI agents using special equipment. The North Koreans, for their part, don't abuse the "privilege" and limit the counterfeiting to a few hundred million dollars a year, which is not enough to really aggravate the US into acting.
If you ever wondered how Kim Jong Il could afford to spend millions each year on cognac, now you know the rest of the story.
See also: http://business.time.com/2012/02/24/how-the-u-s-could-pressure-north-korea-tomorrow-quit-the-100-bill/