The climax of this movie involves a dance sequence on stage. At the end of a dance, one of the dancer pretends to kill another with a spear, but the spear actually goes through a panel on the stage floor. Unbeknownst to the dancers, another woman has crawled under the stage, and she gets killed by the spear. When she screams, the audience boos. The make dancer asks the female dancer why she screamed and ruined their dance, but she claims that she did not scream. I think that leads to the discovery of the dead woman under the stage. I saw it as a child on TV, but I think it was a movie, probably the Saturday night horror movie. This would have been in the 70s.