The administration's 'justification' for the war in Iraq (which, though unspecified, lingered murkily in the penumbra between weapons of mass destruction and a connection between Saddam and Al Quaeda) has of course been conclusively proven false. But that doesn't stop many people from believing it, and it doesn't stop Dick Cheney from bringing it up, even though he knows it's a lie.
Strikes me that this isn't the first time we've done this. I see similarities to the Gulf of Tonkin in 1964. LBJ claimed that US ships had been attacked by the North Vietnamese. Congress believed him and passed the Gulf of Tonkin Resolution, which was the legal justification for the war.
The problem was that there was no attack. US ships were shooting at nothing and then observing the explosions caused by their shots. The evidence was ambiguous then, but the administration kept insisting that the attack happened. Old myths die hard--in his generally fabulous book, MacNamara insists that the evidence was ambiguous, even though it wasn't then and isn't now.
Iraq is a roughly similar situation. In both cases, the administration took some ambiguous (at best) evidence and kept insisting it was true, parlaying it into congressional support. Even afterward, people still believe the lie. So far, we've avoided huge casualties in Iraq, but we were in Vietnam for eleven years after that resolution. Do we want to be in Iraq until 2013? I sure hope not.