During a 1672 production of Macbeth, the actor playing Macbeth got into an argument with the actor playing Duncan. During the play, switching a real dagger for the prop one, he killed him on-stage.
Drunk audience members heckled the actors during a 1721 performance. Finally fed up, the actors leapt off the stage and attacked the hecklers with their swords.
In 1849, a performance of Macbeth in New York ended in a riot. 31 people were trampled to death.
In 1937, Laurence Olivier's sword broke. A piece flew out and killed a man in the audience.
Over the course of a 1942 production, three actors (Duncan and two witches) died. The costume designer committed suicide surrounded by costumes of witches and demons.
In 1948, Diana Wynyard, playing Lady Macbeth sleepwalking through the castle, accidently walked off the stage and died.
Macbeth is cursed. This is largely regarded as fact among theater people. Even saying the name or quoting lines outside of a actual performance or rehearsal is asking for trouble. Instead, it's referred to as "The Scottish Play" or "The Bard's Play."
If you do say "Macbeth" accidentally, the curse can be counteracted by quickly quoting from another Shakespeare play (Hamlet's "Angels and ministers of grace defend us" is a favorite.) Another method is to leave the theater, perform various combinations of spinning around, swearing loudly, and/or spitting over your shoulder, then asking to be let back inside.
There's several explanations why the play is cursed. One is that Shakespeare, in an effort for verisimilitude, used real magical chants for the spells the witches' perform onstage. The Scottish Gallovidian Encyclopedia, an encyclopedia of Scottish culture and folklore gathered by John MacTaggart in the early 19th century, contains an entry for "cantrips," or short black magic incantations. It says, "Various snatches of cantrip rhyme are yet afloat in the atmosphere of tradition, no unsimilar to what Shakespeare introduces in his tragedy Macbeth. Surely, the mighty bard of nature had been no stranger to cantrips with his 'Toil and trouble, toil and trouble / Fire burn, and cauldron bubble.'"
Another theory is that witches cursed the play as revenge for Shakespeare turning their secret rituals into popular entertainment. Still another is they cursed the play to get back at Shakespeare's patron, King James I. It's largely accepted that Macbeth was meant to flatter James. (One of the heroes of the play, Banquo, being a real-life ancestor of his.) James was also an enthusiastic persecutor of witches and even wrote a book on how to find witches. Taking a play meant to glorify the king and add to his legacy and turning it into a disaster-prone folly might have been seen as the perfect way to strike back.
According to the website The Lone Conspirators, a coven of modern-day witches tried to lift the curse permanently:
Several witches had planned on arriving at the old Inverness Castle to reflect positive energy on Macbeth's spirit, but a number of them did not even make it. When the pet dog of one of the witches died, they thought of it as a sign of bad luck and decided not to test fate. Another bad omen, a cat bringing in a black feather, convinced another witch to stay home. In the end, only two witches showed up for the ritual, and even they experienced some difficulties. Witch Kevin Carlyon said "We almost got run off the road coming back from a trip to Skye, and then the three witches who were destined to come up here from other parts of the country all had different individual problems." It was began when Carlyon summoned the four elements of earth, air, wind and water. But when his colleague Eileen Webster, a medium, tried to contact the spirit, she collapsed and began babbling incoherently. Afterwards she said "I sensed a great power that just drained away all my energy. I remember feeling fear. I sensed a very, very evil spirit. I believe in this curse definitely now." She also mentioned that a black crow had stalked her that morning. Despite the setbacks, the witches were able to perform the ceremony eventually, which Carlyon said he thought was a success. "We have reflected the curse, but it will only be when people start saying "Macbeth", and putting on productions of the play, that we will know we have been successful. We won't know until people tell us" were his exact words.
(Cross-posted on my blog.)