I know the World Trade Centers couldn't have been brought down by the US government because the government couldn't possibly keep something that big secret. Think about it, have you ever heard of the government being able to keep a secret like that?
(Think about it.)
(Of course I don't actually believe the conspiracy theory. But I do think it's a funny line of reasoning people use to argue that the government couldn't possibly keep it secret. I have a modicum of faith in our government -- both in its capabilities and its intentions.)