Shakespeare had wanted to impress King James (who had written about demons and witchcraft) during Macbeth, so he used some of the lines from the King's book. People believe that they must have been real witch incantations, because several "accidents" and unfortunate incidents began happening whenever the play was performed. Some of the accidents were real swords being replaced for fake ones and actors getting injured or dying, heavy weights being dropped near actors, actors falling from sets and being injured, riots breaking out at the theatre, etc. Many actors take this curse very seriously and refuse to even say the name of the play in a theatre.
