The ringleaders were the mezzo-soprano Susan Graham and the tenor Paul Groves, who had spent the day rehearsing Lehar’s “The Merry Widow,” which runs at the Met through Jan. 11. In recent years they have made a habit of organizing let-your-hair-down karaoke nights with castmates around their appearances together in cities including Paris, Chicago, San Francisco and New Orleans.

The singers found themselves trading lines in “Bohemian Rhapsody” with the ebullient tenor Barry Banks and rocking out with air guitar solos that would have been right at home in the movie “Wayne’s World.” When the bar door opened and the mezzo-soprano Jamie Barton (currently starring in a revival of “Norma”) walked in, they pulled her to a microphone just in time to join them for the song’s melancholy “nothing really matters” finale. They basked in the whoops and cheers of colleagues and strangers alike.