Angels and ministers of grace defend us!

Wiki:

The Scottish Play and the Bard's play are euphemisms for William Shakespeare's Macbeth. The first is a reference to the play's Scottish setting, the second a reference to Shakespeare's popular nickname. According to a theatrical superstition, called the Scottish curse, saying Macbeth inside a theatre will cause disaster. A variation of the superstition forbids direct quotation of the play while inside a theatre.

Because of this superstition, the lead character is most often referred to as the Scottish King or Scottish Lord. Sometimes Mackers is used to avoid saying the name, mostly in North America.

Those who believe in the curse claim that real spells are cast in the three witches scene. Some believers claim that including the character Hecate, frequently cut from productions of the play due to questions about her part's authorship, intensifies the curse.

Productions of Macbeth are said to have been plagued with accidents, many ending in death. According to legend, this dates back to the premiere of the play: an actor died because a real dagger was mistakenly used instead of the prop^{[dubious – discuss]}. The play does include more fight scenes and other such opportunities for accidents than does the average play, and the atmosphere in the backstage area of old-fashioned theaters was a prime setting for disasters, especially when dealing with potentially dangerous equipment. This would explain the accidents without invoking magic.

The popularity of the superstition might also be related to its mild hazing aspect. Veteran actors might relate some tale of woe that they witnessed personally due to someone invoking the curse, lending credibility and immediacy to the tale.

One hypothesis for the origin of this superstition is that Macbeth, being a popular play, was commonly put on by theatres in danger of going out of business, or that the high production costs of Macbeth put the theatre in financial trouble. An association was made between the production of Macbeth and theatres going out of business.

According to the superstition, Shakespeare got a few of the lines from an actual coven of witches and when they saw the play they were greatly offended and cursed the play. Another tradition tells that the original propmaster could not find a suitable pot for a cauldron and stole one from a coven, who then cursed the play in revenge for the theft. It is believed that breaking the taboo calls the ghosts of the three witches to the show and it is they who cause all the mishaps. The last, and probably most spectacular view of the curse is that Shakespeare used the curse in the play to actually curse the play himself, guaranteeing that no one other than himself would be able to direct the play.

Another line of thought is that if your play was a flop, the manager of the theatre would take the show off and would always be able to get a theatre company to put on Macbeth as it was always a hit.

When the name of the play is spoken in a theatre, tradition requires the person who spoke it to leave, perform traditional cleansing rituals, and be invited back in. The rituals are supposed to ward off the evil that uttering the play's name is feared to bring on.

The rituals include turning three times, spitting over one's left shoulder, swearing, or reciting a line from another of Shakespeare's plays. Popular lines for this purpose include, "Angels and ministers of grace defend us" (Hamlet 1.IV), "If we shadows have offended" (A Midsummer Night's Dream 5.ii), and "Fair thoughts and happy hours attend on you" (The Merchant of Venice, 3.IV). A more elaborate cleansing ritual involves leaving the theater, spinning around and brushing oneself off, and saying "Macbeth" three times before entering again.