When Harry cheated on Sally he accused Sally of cheating. What? It is called projection. Psychological projection is a "psychological defense mechanism in which the human ego defends itself against unconscious impulses or qualities by denying their existence in themselves and attributing them to others.” Projection is a method of blame shifting - Harry blamed Sally for what Harry did.
When Harry accuses Sally it is incumbent upon Sally to defend herself from the bogus charges. This cunning maneuver shifts the focus from Harry to Sally which is the underlying strategic objective of the false allegations. So, while Sally is busy defending herself Harry’s guilt goes unchallenged and he can continue to cheat. The best defense is an offense.