You drop an inflammatory (but totally fictitious) "fact" that supports a particular ideological viewpoint and the dialog has to stop completely while someone goes to the trouble of doing the tedious research and debunking it. By the time the truth is isolated, the lie has caught on and taken off with a life of its own.

A more subtle variation is to point out an inflammatory fact that is nominally true, but taken out of context, carefully selected, or otherwise open to interpretation. This forces a complete literature search and analysis by an expert to debunk it, and the debunking itself can always be criticized (sometimes with some justification)as partisan. OF COURSE ITS PARTISAN! Why would anyone take the time to research an issue unless he had an ideological point to make himself?

A more elaborate variation of the same theme is to build up a detailed case, with charts, graphs, footnotes, testimony and affidavits from credible sources. But lawyers do this all the time, and their opponents in court put together very similar cases to counter these briefs. It takes a skilled and honest judge to get to the bottom of these controversies and we know for a fact that even they don't get it right all the time. Its what lawyers and propagandists and marketers (and politicians) do, they lie for a living, and they are proud of it.

Even honest, competent scholars, journalists and scientists look at the same evidence in non-controversial issues and sometimes arrive at different conclusions. And I'm not even talking about the philosophers and theologians.

Politics is not physics. There is no absolute Truth in the social sciences, no correct way to interpret the historical record, no "fact" which can establish a political or economic theory is right or wrong. Like the lawyers in a complex civil case, you can always shuffle the evidence around to prove anything you like.

Look at it from the point of view of a more "objective" discipline: Military Science. (After all, one side wins and the other loses, right? You can't get any more objective than that.) One man's brilliant "pincer movement" is another's "prematurely dividing his forces in enemy territory". One general "commits his reserves too early", another waits too long. One defense is called "dogged and determined", the same tactic used by another army is criticized as being "rigid and inflexible". Even the objective measure of victory or defeat is open to interpretation. The Battle of the Coral Sea was a tactical victory but a strategic defeat for the Imperial Japanese Navy. General Giap defeated the world's strongest military after losing every major battle he fought with it. And every loser has his excuses, "we were stabbed in the back", "we were betrayed",
or my favorite, used by both Hitler and Bonaparte, "It snows on the enemy, too."

The point is, there is no absolute, final Truth, maybe not even in physics. The best we can do when facing the real world, confronted with insufficient data, confused perceptions and conflicting interpretations, is to skew the odds in our favor a little bit. Anything else is hubris and arrogance.