The exploitation of the South Wales Coalfield, particularly from the last quarter of the nineteenth century onward, was accompanied by numerous instances of instability within spoil heaps and their foundation materials. Sudden failures which were sufficiently rapid to overwhelm property and services and, in some instances, to threaten life, occurred on at least twenty three occasions, details of which are provided. It is believed that thesel included sixteen flow slides, five debris slides and two failures caused by outbursts of groundwater. At five sites, debris flow was a secondary failure mechanism. The locations of these failures are mostly clustered in those parts of the coalfield with the highest relief and with the highest rainfall, although antecedent rainfall conditions for the failures were variable. Most are shown to be associated with active tipping faces, but one flow slide is believed to have occurred on a tip four years after its abandonment and an outburst failure on a tip fifteen years old. The occurrence of rapid failures is shown to mirror the development of the coalfield but has ceased largely as a result of legislation to improve tipping practice, which was enacted following the Aberfan flow slide.