In addition to the rope breaking in Ellisons near the warm up pit, there was also an incident at Megawell in 1997.
Three cavers were using a Polish caving rope, which was cut by rockfall. Unfortunately causing one of the cavers to fall to his death. The rope was tested afterward and despite being 10mm, was about half the strength of PMI 10mm. It was also subject to high elongation for a static rope. The caver that died was heavier set and was frogging, causing sever elongation on the rope, which lead to being easily cut by a falling rock.
(The popular version of the story I have been told numerous times is that the rope was cut at the edge, while it may have been a contributing factor, the investigation afterward suggested rock fall.)
Read more on these incidents in the ACA archive (login is NSS number & zip code):
https://secure.caves.org/nss-business/p ... ndex.shtml