Although the scene resembled a Marx Brothers movie, there was nothing funny about the multiple emergencies almost capsizing the San Francisco Opera's "Simon Boccanegra" in the War Memorial on Friday. To begin with, Paolo Gavanelli, who sings the title role, had the flu, a bad case. He went on anyway, but couldn't finish the performance. When the call went out for a "baritone in the house," there was just one available, Boccanegra's deadly enemy, Paolo, sung by Nikolai Putilin. So, Putilin took over the role of the man he would have poisoned later, and the role of Paolo went to Patrick Carfizzi - all the role changes and role reversals happening during intermission. At the same time, Carol Vaness had an asthma attack, but she completed her assignment as Amelia, and few in the audience suspected the circumstances she was fighting vocally, never mind the fact that her former nasty suitor became her long-lost father, giving Verdi a new kind of psycho-sexual complexity. Raymond Aceto, singing Pietro, had a throat infection and he too would have quit during intermission if only all the other problems didn't throw the evening into turmoil. At the Sunday performance, Putilin continued as Boccanegra, Carfizzi singing Paolo through the full opera. Gavenelli is expected to return on Wednesday. Janos Gereben/SF [log in to unmask]