Theatre's such a precarious, sometimes inexplicable business that it's no wonder its participants often turn to superstition to make sense of it. Psychotherapists might say this stems from a sense of projection and denial: when bad things (and bad productions) happen, it's easier to blame unseen external forces rather than accepting responsibility for your own part in such accidents and failures.
And when some plays like Macbeth prove to be particularly intransigent beasts to put on, isn't it convenient to have a treasure trove of superstition to draw on? In theatrical circles, Macbeth is duly seldom referred to by name - and never so backstage - but rather by the moniker "The Scottish Play"; and, if the name is accidentally spoken in a dressing room, tradition requires the speaker to leave the room, turn round three times anti-clockwise (as if to turn back time), and then knock on the door three times to request permission to re-enter.
While the bad luck associated with Macbeth is often attributed to some mysterious quality associated with the witches' incantations that open the play, it was actually the tragedy's popularity with the public - and the fact that managements down on their luck would therefore turn to it whenever their seasons were flagging - that meant it has paradoxically come to be associated with failure. If even Macbeth might not be enough to save them, the play would frequently portend the company's imminent demise.
The superstitions around the piece, being put on out of practicality, have been partly borne out by experience: so, instead of trying to counteract failure, it nowadays seems to invite it. Legend, too, has it that, at the first-ever performance in 1606, the boy actor playing Lady Macbeth (or Lady M, as she's referred to, in an attempt to avoid the M word) was taken ill before it began and died before it was completed. Whatever the case, Macbeth continues to be regarded as an unlucky play.
Meanwhile, "I Dreamt I Dwelt in Marble Halls", a song from a musical called Bohemian Girl, is regarded as an unlucky tune - again, legend has it that it was being played aboard the Titanic at the time it struck the iceberg, so is banned backstage. Also on the prohibited list: "Three Blind Mice".
Cats & peacocks
Cats, on the other hand, are both lucky (especially when it comes to the musical named after them!) and unlucky. It's considered good luck to have one in the theatre (and many still have their resident pusses, to ward off the aforementioned mice, sighted or unseen!) - but if one runs across the stage during a performance, bad luck is expected to follow.
Other animals to avoid: peacocks, or at least their feathers, since the 'eyes' on the feathers have become associated with the Evil Eye, a symbol of misfortune. Colours to avoid: blue and green. Blue, because of the expense of the dye used to create it, suggested that the company were overspending and therefore were sure to close prematurely; green, because limelight (in pre-electricity days) created a greenish hue and might render the wearer of a green costume invisible or ghost-like.
Also to avoid: bouquets of flowers before a performance could be an omen of failure (but presented afterwards are very welcome); the use of real flowers onstage (though this of course could be merely a budgetary consideration so that they don't need to be replaced frequently); and burning candles (again probably born of pragmatism, because costumes and freshly painted scenery are liable to catch fire). It's also considered unlucky to use real mirrors or jewellery onstage. Practicality dictates that the reflective qualities of these could disturb lighting designs, but the superstitious actor also considers it unlucky to look into a mirror over the shoulder of another (so that the two reflections are seen together) as it's thought to bring certain misfortune to the one overlooked.
Make-up and costumes create numerous grounds for superstition. The upsetting of a make-up box, a box with a new set of greasepaints on an opening night, and the cleaning out of boxes are all said to presage bad luck. Logical reasons are easy to find: all sorts of nasty stuff can get into make-up that's been on the floor; new make-up that hasn't been previously tested might melt under the lights; and the moment you throw anything away is of course when you need it. Some actors, too, won't unpack their make-up boxes before the reviews appear - to do so might tempt fate that they expect a long run. As for costumes, any loose threads should be removed not by the wearer but by another actor, then wound around a forefinger; the number of revolutions the thread requires is meant to indicate how long the run might be!
Whistling & breaking legs
More pertinent hazards are avoided by the superstition against whistling onstage: this is owing to the fact that, prior to the advent of remote communication methods via headsets and the like, stage managers would signal their scenery cues to the flymen above the stage using sailor's whistles. An onstage whistle could therefore set the scenery in motion before it's due. Whistling in a dressing room is also to be avoided, as that's said to conjure up an ill-wind that could result in a short run. Knitting onstage or in the wings is also prohibited by superstition, again probably for practical reasons: a pointy needle could rip a costume as it goes by to make an entrance.
Wishing an actor "break a leg", on the other hand, is not an invitation to suffer ill-fortune but precisely the opposite: you must never wish an actor good luck before a performance. Luck is apt to fill them with undue confidence; but breaking a leg is thought to derive from a military term for bending down on one knee and breaking the line of the leg, or in theatrical terms, to taking a bow - the reward at the end of the performance.