Yeah I know there are some examples of conspiracies which were successful, but were found out (or were common knowledge), I was really making a point about reasoning about the unknown from just knowns. You can't do it, because the set of unknowns by definition has different traits to the set of knowns.
Actually one of my programming lecturers points out the only decent thing Dick Cheney said. Paraphrasing: "there are known knowns, and known unknowns, and you can reason about those and plan for them, but there are also unknown unknowns, and you can't plan for those". In the realm of conspiracies, we have things that are known to have happened, things we suspect may have happened, and a mystery class of things that happened which nobody even suspects. Naturally, we can save very little about the last class of conspiracies, and we definitely can't draw conclusions about them from the known ones.
It's similar to serial killers. What we know about them is derived from the ones who've been caught or otherwise found out, left evidence or bodies. But there are a lot more missing people out there, and there could be entire classes of serial killers out there with methods we don't even known about. So when we say authoritatively that serial killers have this or that personality, that they tend to use this or that method, all that's derived from the study of ones who got caught or detected, never ones who properly covered their tracks. Are we developing the right methods to catch those people? We really can't say, since what we know is derived from studying the failures.