My university training was in history, and one valuable thing I got out of four years of study was a life-long skepticism about historical writing, including the off-the-cuff contemporary history known as journalism.
History is a selective retelling of events based on very imprecise and incomplete sources. Quite a shocking amount is sheer guess-work. Historians quote each other and thereby enshrine certain memes as "facts."
One obvious bias in all the sources is that history is always written by the victors. Our concept of the Punic Wars would be quite different if the Carthaginians had won. Portrayals of the enemy are always skewed. Usually they are demonized, but occassionally they are given the status of tragic noble savages, as in Tacitus' portrayal of the Germans. This is also for political effect, Tacitus was a conservative who wanted to contrast the noble simplicity of the Germans with the hedonistic decadence of the Romans.
History is very often used and abused to serve contemporary political ends. Every time politicians want to drum up support for a war, they are sure to dredge up the hoary example of the Munich conference of 1938. If anyone took the trouble to really read up on 1938 they'd understand that it should serve as an example of the folly of international acceptance of aggression, it should be a pacifist example. (At least that's how I'd spin it)
Popular presentations of history are especially suitable vehicles for propaganda. Two recent movies (confession; I've not seen either) seem to make the case. Both are very inaccurate representations of the past, both have fairly blatant agendas relevant to contemporary issues.
Mel Gibson's Apocalypto is a statement for aggresive and expansionist Christianity (Mel belongs to a fundamentalist sect more Catholic than the pope). It misrepresents Mayan culture (denies it actually) and portrays the Spanish conquest as the coming of light into the heathen darkness. Link.
The movie about Sparta, 300, sounds like a ridiculous caricture. The political agenda of a movie about heroic good-guys fighting for "freedom" against evil swarthy Persians (no less) is too obvious in the current climate to need explaining. But if you must, go here Link. What troubles me is that contemporary Americans identify more easily with the Spartans than the Athenians.
Then there are those cases in which the interpretation of history becomes a hot political issue in it's own right. The Canadian War Museum is under fire from veteran groups for suggesting in their display on strategic bombing that German civilians were killed by Canadian air-men. Heavens, we can't let facts interfere with the myth. Link.
Finally, if you want to see just how shaky the whole edifice really is, consider the new school of maverick historical researchers, mostly Russian, who claim all history before about 1500 really is bunk. They call themselves the New Chronologists and claim, basically, that the Middle Ages never happened. According to them, about a thousand years of pure fiction got tacked in for political purposes during the Renaissance. Wild stuff, to be sure, but how do we really know?
(I think the numismatic evidence refutes them, but it's fun to play with)