It's 'Macbeth' which is traditionally bad luck to mention by name. Actors are a notoriously suspicious lot, and this strand of superstition comes from the days of Rep. Theatre. If a play was doing badly, and due to be taken off before the end of its run, the theatre would often fill the remaining time with performances of 'Macbeth', and this evolved into the notion that mentioning the name meant that the current play was due to fail, so no-one was supposed to refer to it.If you do want to refer to 'Macbeth' in theatrical circles, and spare the feelings of any thespians, refer to it as 'The Scottish Play', everyone will know what you mean.