Tradition says that the show must go on , but what about when the show must go off ? It eventually did at Tuesday night's performance of "The Phantom of the Opera" at the Ahmanson Theatre, but not until after the curtain call was stalled for several puzzling minutes. When the show ended, the curtain came down (as usual), the orchestra played (as usual), the audience applauded (as usual), but the curtain stayed down (not as usual). With the curtain still down, the applause eventually stopped, leaving the audience to wonder in silence what was holding things up, or in this case, holding the curtain down. The problem was with the portcullis that encloses the Phantom's lair. All movement of set pieces is computerized, and when the portcullis moved too soon, explained a spokeswoman for the show, it had to be stopped and the computer reset. The delay also was a safety measure to ensure that the gendarmes climbing the portcullis at the end of the show were properly secured. They were, and after one more fumble, the curtain did go up and the actors were able to take their delayed bows and receive their ovation.