Superstition

Before closing the posts on Macbeth, I would like to cite a couple sources that give a reason for the theatrical superstitions on the Scottish Play, here is the second:
http://en.wikipedia.org/wiki/Theatrical_superstitions

Shakespeare's play Macbeth is said to be cursed, so actors avoid saying its name (the euphemism "The Scottish Play" is used instead). Actors also avoid even quoting the lines from Macbeth inside a theatre, particularly the Witches incantations. Outside of a theatre the play can be spoken of openly. If an actor speaks the name Macbeth in a theatre, he is required to leave the theatre building, spin around three times, spit, curse, and then knock to be allowed back in. There are several possible origins for this superstition. One is the assumption that the song of the Weird Sisters is an actual spell that will bring about evil spirits. Another is that there is more swordplay in it than most other Shakespeare plays, and the more swordplay must be rehearsed and performed, the more chances there are for someone to get injured. Yet another option is that the play is often run by theatres that are in debt and looking to increase patronage.