Definition: A performance fails or does not go as expected (disruption, inability to sing or dance, technical incident, need to improvise or to catch up during the performance…). Includes scenes in which a performer makes a fool of himself. The performance and the show go on (doesn't include numbers that stop when disturbed).
Example: “You And The Night And The Music” (Band Wagon); “Pettin’ In The Park” (Gold Diggers of 1933); “Gotta Have Me Go With You” (A Star is Born). Not just in the case of backstage musicals (“I Have Confidence” in Sound of Music)