Reichstag Fire
The Reichstag was the building that housed the German parliament in 1933. It was set on fire in 1933.
A single Dutch communist who acted alone was likely responsible for it. He was convicted and executed for the crime.
The significance of the event is that Adolf Hitler claimed it was an organized plot by the entire Communist movement. He used the event to suspend civil liberties and launch a campaign against communism, which eventually gave rise to the Nazi party.
Colloquially, the event can be used to represent something that is overblown or misrepresented to justify larger measures to supposedly prevent it, but which are actually in pursuit of a larger, less acceptable goal.
Why I Looked It Up
In Conspiracies and Secret Societies, various theories around 9/11 are discussed:
The events of 9/11 provided the [New World Order] which their Reichstag fire – a cowardly attack against innocent American citizens that would justify declaring a war on terrorism and a siege against the accused architects of the sneak attacked, Osama bin Laden and Saddam Hussein.