When Ellen Naomi Cohen (aka Mama Cass) first tried to join John Phillips's band, then known as the New Journeymen, Phillips rejected her because she couldn't hit the highest notes. Shortly thereafter, Cass's voice changed, Phillips changed his mind, and the Mamas & the Papas were born.
So what prompted Cass's voice to change? Did she go on a diet to lose some weight? Not at all. While walking by a construction site one day, she was knocked unconscious by a falling pipe and when she recovered, she was able to sing in a higher range!