Introducing Shakespeare: A Graphic Guide - Nick Groom, Piero 2013
The Curse of Macbeth
There was also a darker theatrical tradition: the curse of Macbeth. This arose with rumours that the witches’ spells were authentic magic. Early performances discovered actors really murdered on stage or in danger from collapsing scenery, or inexplicable fatalities among actors and stagehands.
PRODUCTIONS WERE GENERALLY DISRUPTED AND OFTEN NEVER RECOVERED. EVEN MENTIONING THE NAME OF THE PLAY IN A THEATRE IS BAD LUCK.
Actors cross themselves or quote from The Merchant of Venice: “Fair thoughts and happy hours attend on you!” (III.iv.41) Or, more elaborately, turn around three times, spit, knock on the dressing-room door three times, and plead to enter.