If you repeat something over and over, no matter how outrageous it may be, people will come to believe there's some truth in it. A good example of this is the claim that Saddam Hussein was responsible for the terrorist attacks of September 11, 2001. No evidence has been found suggesting collaboration between Iraq and the Al Qaeda network, yet Bush administration officials have repeatedly mentioned the two in tandem. As a result, a recent opinion survey by the Council on Foreign Relations shows that more than 40 percent of the American people believe that some or all of the attackers on 9/11 were Iraqi nationals, when in fact none were.
Sometimes old propaganda has a way of haunting its perpetrators. In the late 1980s, for example, the United States regarded Iraq as an ally in its ongoing conflict with Iran, even as reports emerged that Saddam Hussein had used chemical weapons against his own citizens - Iraqi Kurds at the town of Halabja. The U.S. at the time argued that Iran was responsible for the atrocity, and the controversy continues today, even though the United States now officially insists that Iraq was responsible. According to Stephen C. Pelletiere, the facts surrounding that claim have been selectively presented and distorted. "I am in a position to know," he stated in the New York Times, "because, as the Central Intelligence Agency's senior political analyst on Iraq during the Iran-Iraq war, and as a professor at the Army War College from 1988 to 2000, I was privy to much of the classified material that flowed through Washington having to do with the Persian Gulf. In addition, I headed a 1991 Army investigation into how the Iraqis would fight a war against the United States; the classified version of the report went into great detail on the Halabja affair." 
Kenneth Roth of Human Rights Watch, which has conducted extensive investigations into the Halabja affair, insists strongly that Iraq was responsible for the incident, yet the controversy continues and may never be completely resolved.  Instead of leading to definitive answers, old propaganda continues to be repeated long after it has outlived its usefulness to the propagandists.
Some interesting (and scary) quotes on repetition:
"But the most brilliant propagandist technique will yield no success unless one fundamental principle is borne in mind constantly and with unflagging attention. It must confine itself to a few points and repeat them over and over." -- Adolf Hitler, Mein Kampf, p. 184
"The purpose of propaganda is not to provide interesting distraction for blasé young gentlemen, but to convince… the masses. But the masses are slow moving, and they always require a certain time before they are ready even to notice a thing, and only after the simplest ideas are repeated thousands of times will the masses finally remember them." -- Adolf Hitler, Mein Kampf, p. 185
Quotes from Mein Kampf, trans. by Ralph Manheim, (Cambridge, Mass.: The Riverside Press, 1962).