Conspiracies

While discussing Rosie O'Donnell, Vincent Bugliosi and others, I heard the following argument: big conspiracy theories can't be true because it's impossible for even a small group of people to hide all that evidence for a long period of time--no one can name a single case where this has been pulled off.

That it's hard, maybe impossible, to get away with a huge conspiracy may be true, but this argument has an obvious flaw--if anyone did get away with such a thing, we wouldn't know about it.