Merry Clayton, who sang backup vocals on "Gimme Shelter," sang (screamed more like it) so hard that her voice actually cracks in final stanza (if you listen closely you can hear it). She was pregnant at the time and allegedly miscarried a couple days later from straining so hard on the vocal.
There was an interesting documentary that I saw where Mick Jagger says that he felt bad that they got her up at 2am, her friend and producer literally dragging her out of bed, to sing what at the time were pretty controversial and raunchy lyrics: "
****, murder, it's just a shot away!" Singing about
**** and murder, bullets and semen, like that in the 1960s was more or less unheard of.
Gimme Shelter is literally a moment in rock history and her vocals are a shining example of an approach to rock music that has long since died. The Great Gig in the Sky is another example of a classic rock song with amazing backup vocals by Clare Torry. It's a lost art.