Last month, it happened to U2 in Mexico City. It also happened to Shakira:
Recently, Lady Gaga encountered a fan gesturing along with a song on stage in Japan:
Brittany Spears had something similar happen the year before:
Of course, there's the famous "Soy Bomb" incident during the Grammy Awards telecast in 1997, when performance artist Michael Portnoy jumped up next to Bob Dylan, during a "comeback" performance, and danced for at least a minute before being escorted out by security:
The best part of that Grammy moment is Dylan's nonchalant reaction. He basically just shrugs and keeps on playing. As a student of American popular culture, I'm sure that Dylan knew that this sort of thing has had a long history. Before middle-class reformers in the 1860s and 1870s made theaters and concert halls safe for the inner contemplation of works unfolding uninterrupted onstage (see John Kasson for more), audiences regularly interrupted performances, grabbed at actors, roamed the stage, and demanded repeated encores. Richard Butsch (in The Making of American Audiences, pg. 50) noted a dispatch from the New York Mirror in 1832 describing a performance of Richard III at the Bowery Theater:
The boys [on stage] frequently ran between King Richard and Lady Anne to snatch a stray copper. In the tent scene, so solemn and impressive, several curious amateurs went up to the table, took up the crown, posed the heavy sword, and examined all the regalia with great care...When Mr. Rice came on the stage to sing his celebrated song of Jim Crow, they not only made him repeat it some twenty times, but hemmed him in so that he actually had no room to perform the little dancing or turning about appertaining to the song; and in the afterpiece, where a supper-table is spread, some among the most hungry very leisurely helped themselves to the viands.Sounds like today's artists have it pretty easy.