One of the pleasures of summer in New York is the Shakespeare festival at the Delacorte Theater in Central Park, where the plays are performed outdoors, free of charge. A few nights ago I went with a friend to see “Twelfth Night,” in which Oliver Platt was brilliant, Jimmy Smits was fine and Julia Stiles was disappointing. The evening began with an announcement, as customary as the national anthem before a baseball game, that all cellphones must be switched off. Midway through the second act a phone rang behind me; instead of turning it off in an embarrassed rush, a woman began chatting with, it seemed, her babysitter. She ignored the shushes of everyone around her and disturbed the play. Tragedy turned to farce as she loudly threatened to summon security if the attempts to silence her continued; this was odd, because we wanted to scramble security against her. But Shakespeare might not have minded the interruption, which was a reminder, like the play before us, of the perseverence of human folly.