A non-sequitur for sure, but the volume of - often declassified - such acts from the U.S. government often makes it hard to dismiss conspiracies out of hand.

Operation Northwoods is often cited because of how impossibly evil it is - that it was dismissed by the president should be noteworthy less for its dismissal and more for the fact that it made it to the president's desk.

There are obviously others, like the Tuskegee syphilis experiment, for example, that demonstrate that large agencies sometimes work outside the boundaries of logical, empathetic human response.