From the journals of Sir George Smart

They settled the manner at rehearsal as how it was to be sung, but when the time came, Madame Caradori-Allan made some deviations; this prompted Malibran to do the same, in which she displayed a most wonderful execution. During the demanded encore, [Maria] turned to me and said, "If I sing it again it will kill me". "Then do not," I replied. "Let me address the audience." "No," said she. "I will sing it again and I will annihilate her." She was taken ill with a fainting fit after the duet and carried into her room.