What would be examples of historical evidence that support Voltaire's statement, "It is dangerous to be right in matters about which the established authorities are wrong"?
I think that the fundamental issue in Voltaire's support speaks to the idea that established authority can be wrong at moments, and their exercise of power in such instances has to be deemed as wrong. The challenge is that speaking out in such instances can be disastrous. Historical examples of this can be seen in Voltaire's own historical context. Speaking out against the French Monarchy and its abuse of power would have been one such historical example where it would have been "dangerous" to be right when "the established authority was wrong." The converse historical reality would have been equally true, also, in that the Reign of Terror in the French Revolution was an instance when the revolutionaries were wrong and speaking out against their abuse of perceived justice would have been right, yet spelled out disastrous consequences. Modern historical examples would include Hitler and the Nazis, an instant when speaking out would have been right, but carried with it undeniable consequences. Stalin's reign of the Soviet Union would have been another instance where Voltaire's statement would have been proven true with brutal consequences. All of these historical examples would have proven Voltaire being right in his mistrust of centralized authority structures.
There are many literary/historical evidence that could support that statement. The one that comes to mind is the case of Jean Calas.
Jean Calas was a protestant with a son who was converting to Catholicism. The son was found dead and the town of Toulouse immediately blamed Jean for his sons death because he wanted to prevent his son from practicing Catholicism. Jean Calas was tried in court and sentenced to death. After his death Voltaire convinced many that the sentencing was biased because the judges were all Catholic. The sentencing was reviewed and taken back. In reality, Jean Calas' son had committed suicide. After the ordeal Voltaire published a very controversial book called Traité sur la Tolérance.
One could argue that because of Voltaire's activism in searching for justice in Jean Calas' case, he ran the risk of facing dangeorous consequences. 18th century France was reigned over an absolute monarchy and anything or anyone that went against the morals of king and country would face punishment. It was also extremely dangerous for Voltaire, notorious with the French authorities, to publish Traité sur la Tolérance as he openly criticized the case of Jean Calas, the prejudice against Protestantism and France's corrupted judicial system.