Group of 3 abseil down Lumbar Puncture from the ab steak at the top. Two ropes joined using back to back overhand knots, well dressed and small. Take off edge was clear of loose debris.
Ropes were unusually difficult to pull (we've been down here before). Needed body weight to move them. Then a block about 2/3 the size of a house brick came over the edge, narrowly missing me, landing about 2m behind.
Ropes were easier after that. I presume the knots had caught on a half buried block and ripped it out, sending it over the edge.