As has been starkly illustrated of late. The outright lying about President Bush's uranium statement is one example.
A good example of the position switching is the revelation that in 1999 media and "intelligence sources" the world over were saying it was likely and probable that Saddam Hussein and Osama bin Laden were forging an alliance. It was reported back then that Saddam offered asylum to bin Laden. Now George W. Bush is portrayed as some kind of crackpot for dreaming up such a possible relationship, when it turns out it most likely (no, it hasn't been proven--yet) was being forged years ago.
Now they are saying the prez has little credibilty and we cannot act (on N. Korea?) on intelligence that lacks credibility.