cross that line and you will never be king thereafter

Rachel Cowan

In the old days, a black cat was often believed to be the familiar of a witch. If such a cat crossed your path, you might well be cursed.

In Verdi's version of The Scottish Play, Lady M, descending into madness, tries desperately to cleanse herself of the blood of the old king.
chi poteva tanto sangue immaginar [who would have thought he could hold so much blood]

Superstition is still very much alive in the theatre and to speak the name of Shakespeare's play (or Verdi's opera) within the four walls of a theatre is to bring down a curse upon the performance.