A study conducted by the Program on International Policy Attitudes (University of Maryland) and the Center on Policy Attitudes found that people whose primary source of news is FOX News were much more likely to hold three demonstrably false misperceptions about the war in Iraq than those who got their news from any other network, and those whose primary source of news was NPR or PBS were far less likely to hold any of the misperceptions. (http://www.pipa.org/onlinereports)
The media is biased politically.
People in the news media make mistakes; they sometimes accept claims with insufficient evidence or without confirming the credibility of a source. The media are also subject to pressure and manipulation from government and other news sources. In addition, the media are driven in part by the necessity to make profit!
Finally, the news media are to a great extent a reflection of the society at large. If the public are willing to "get by with superficial, sensationalist or manipulated news, then they can rest assured that's all the news they will get" (Moore & Parker).