There's a certain etiquette that audience members generally adhere to while watching a live performance, and that goes doubly for the opera world. But you don't have to be an opera-goer to know that it's generally frowned upon—to put it lightly—for a member of the audience to stand up and start singing right in the middle of an opera singer's performance.

It ain't Lollapalooza, for crying out loud.

But an audience member adding his voice to an opera performance was exactly what happened at the Verdi Festival in Parma, Italy this past fall. According to Classic FM, renowned soprano Lisette Oropesa was performing an encore at the end of her recital, singing the female part from "Sempre Libera" (Always Free) from Verdi's "La traviata." Thesong is a duet, usually sung between a female soprano and a male tenor, but she was performing it solo. So when the tenor part arrived and no one sang opposite her, 24-year-old Liu Jianwei, a fan of Oropesa and a student of opera at the Conservatorio Giuseppe Nicolini di Piacenza, stood up and filled in the gap.

No one expected it. Not Oropesa. Not even Liu himself, apparently. But the pianist kept playing and Oropesa appeared to be delighted as the young man beautifully filled in the tenor part. Oropesa's initial "Oh," is written into the piece (though you can see her searching the audience for where the man's voice was coming from), but the "Oh, grazie," she added herself to say thank you.

It's a good thing he had a lovely voice. Watch: