Last month two of my friends were climbing at Skeletal in Spearfish Canyon. The leader was a new leader and was climbing Soiled Dove and fell several times getting through the crux, his gear and belayer held. He finally made it to the anchors and was preparing to clip a draw into the anchors. Somehow he slipped before he made the clip and fell. He said he remembers seeing one bolt and then another go by before he decked, ass first then head. He landed facing the rock and fortunately, other than a sore butt and rope burn on the back of his leg, was OK.
He asked his belayer what happened. All they (who shall remain nameless) said was the rope pulled through their hand.
Several weeks later we figured out what happened. When the leader was preparing to make the clip the belayer fed out slack and let go of the rope entirely to grab lower on the tail. At that exact moment the leader slipped and fell pulling the rope through the belay device.
It is a miracle that he wasn't more injured than he was.
Belayers NEVER let go of the rope with your break hand. When pulling slack move your break hand up to the belay device. Move your non-break hand back down to the belay device. Slide your break hand back down never leaving the rope.When taking in slack, take with both hands and put the leader in break. Move your non-break hand below the break hand. Keeping the leader in break, slide your break hand back up to the belay device and then move your non-break hand back up above the belay device.