Person A was belaying person B. A is a very competent climber and belayer, and was using an standard ATC. B was at the top of the route (10m) and fell unexpectedly. I saw A not have control of the break rope and B started to fall to the ground pulling A into the wall, while the rope was continuing to flow through the belay plate. A managed to grab the break strand at the last minute and B ended up 1m from the floor. It was a very close and lucky call.