From Wikipedia "Conspiracy Theory" category

"Real conspiracies"

On some occasions particular conspiracy allegations turn out to be readily verifiable, as in the French government's attempted cover-up following Emile Zola's accusations in the Dreyfus Affair, or in the efforts by the Tsar's secret police to foment anti-Semitism by presenting The Protocols of the Elders of Zion as an authentic text.[18] Where such success is due to sound investigative methodology, it is clear that it would not exhibit many of the compromising features identified as characteristic of conspiracy theory, and would thus not commonly be considered a 'Conspiracy theory'. In the case of the 1971 revelation of the FBI's COINTELPRO counter-intelligence work against domestic political activists, it is not clear to what extent a 'conspiracy theory' involving government agents was either proposed or dismissed prior to the programme's factual exposure. [citation needed]
Some argue that the reality of such conspiracies should caution against any casual dismissal of conspiracy theory. Many "conspiracy theory" authors and publishers, such as Robert Anton Wilson and Disinfo, use proven conspiracies as evidence of what a secret plot can accomplish. In doing so, they attempt to rebut the assumption that conspiracies don't exist, or that any "conspiracy theory" is necessarily false. A number of true or possibly true conspiracies are cited in making this case; the Mafia, the Business Plot, MKULTRA, various CIA involvements in overseas coups d'état, Operation Northwoods, the 1991 Testimony of Nayirah before the US Congress, the Tuskegee Study of Untreated Syphilis in the Negro Male, the General Motors streetcar conspiracy and the Pearl Harbor advance-knowledge debate, among others.[citation needed]
The argument is often advanced that the non-existence of any given conspiracy is shown by the lack of leakers or whistleblowers. Given the success of the British government in getting thousands of people to keep the ULTRA secret -- and thereby ensuring that no reliable history of World War II could be published until the 1970s -- it is apparent that this is not necessarily a reliable indicator.[citation needed]Machiavelli, stated in The Discourses on Livy that conspiracies rarely achieve their objectives.

[Doesn't mean conspiracies don't exist - the Curmudgeon]