Two incidents of raucously censorious opera audiences seem to have caught the public unaware in recent weeks. At the Metropolitan Opera, Placido Domingo was booed when he conducted a “La Bohème,” in which Anna Netrebko sang her only Mimi of the Met season. Five days later at La Scala in Milan, Italy, Roberto Alagna, singing Radamès in Franco Zeffirelli’s new production of “Aida,” was greeted with catcalls at the end of his “Celeste Aida,” early in Act I. Mr. Alagna beat a hasty retreat, and Antonello Palombi was thrown onstage to pinch hit.

These two episodes generated more press buzz than is usual today for opera, which — like classical ballet — has virtually disappeared from the television screen. And it was the degree of interest that was surprising, given that opera — and to some extent ballet — have a long history of vociferous audience displays. And at no house has this been more true than at La Scala. But the stir created by Mr. Alagna’s walkout and Mr. Domingo’s handling of Ms. Netrebko may be due in part to the fact that in today’s context they are anomalous examples of displeasure: We are living through a period of comparatively sedate audience behavior.