You may hear the term 'False Flag' bandied about - especially on social media - when talking about terrorist atrocities, mass-shootings, assassinations and other exceptional violent events. It is used to dismiss the repercussions that may stem from these events by either sympathisers of the perpetrators or by people who do not want to accept the actions that might be needed to mitigate and reduce these events from happening in future.
Essentially it means claiming that the people who committed the atrocity were not doing so for the obvious, claimed reasons but were actually acting on behalf of their opponents, either to frame the group who actually did it or for some other nefarious reason. For example: 9/11 is often claimed to be an inside job, done by government agents to promote the invasion of Iraq and grab oil revenues for the wealthy elite. Alternatively the victims of school shootings are dismissed as paid actors and the whole thing staged simply to promote gun-control.
Historically, False-flag operations have been committed by secret security services during war-time or to give excuse for war - for instance the Gleiwitz incident which gave Germany an excuse to invade Poland in 1939 - or for political parties to place agent provocateurs amongst demonstrations or other organisations. However, there is good, well- documented evidence for many of these. Conspiracy theorists use the FF gambit without any real evidence as a way to avoid the repercussions, dismiss or excuse contrary evidence to their hypothesis or to promote their own preferred response.