Suggested by lumos47
In the scene, Ellen Burstyn's character was supposed to be thrown to the ground by her possessed daughter. Burstyn was wearing a wire, and to make everything look real, the person controlling the wire would pull on it, which would fling her body to the ground.
Burstyn landed hard on her back during the first take, and the director called to redo it, so she responded, "Billy [Friedkin], he’s pulling me too hard...I know it has to look real, but I’m telling you, I could get hurt." And so he said, "OK, don’t pull her so hard.” Burstyn was apparently pulled just as hard the second time, and that's what hurt her spine. In 2014, over 40 years after filming the scene, she said, "I did injure my lower back and had to work with it ever since. But it’s okay.”