In Act 3, Scene 4, Polonius conspires to have Gertrude speak to her son, Hamlet, while he hides behind a tapestry Hamlet. In doing so, Hamlet assumes that it is Claudius hiding behind the curtain, stabs blindly through the fabric, thinking he is killing the King. This causes Claudius to fear for his own life, Ophelia to go mad, and Laertes to seek revenge, which leads to the duel in the final act.
'Thou wretched rash, intruding fool, farewell, I took thee for thy better’.