One of the most impressive flash floods of the 20th century in France as well as in Catalunia occurred in the Eastern part of the Pyrenees on October 1940. 47 people were killed in France during this extraordinary event and more than 100 in Spain. It caused tremendous damages to buildings and, in particular, destroyed the center of the resort town of Vernet-les-Bains on the slopes of the mount Canigou. The maximum observed 24-hour accumulated rainfall reached locally 1000 mm and this remains until a record value for the French European territory. The flood has been abundantly documented by the technical State services as well as some scientists of the time and a large part of this documentation has been archived. At the light of the recent advances in flash flood studies, this data set has been unearthed and the past analyses of the event have been deeply revisited. This revealed, in particular, that the peak discharge values, on which local risk assessment studies are based, had been largely over-estimated. This led to several mis-interpretations of the processes occurred during this flood such inundation dynamics or the driving factors of the tremendous observed scour and erosion volumes. Some particular features of this event could also be confirmed or revealed: (1) an impressive wave propagation of about four million cubic meters due to the breach of a natural dam due to a massive landslide in the upper part of the Tech river which occurred during the night, affected already destroyed areas and got therefore almost unnoticed, (2) a local major amplification of the inundation induced by the breach of a railway embankment, (3) the dynamics of scour and erosion. Beyond the conclusions drawn on this specific extreme flash flood, this study illustrates that our knowledge about extreme flood events is still limited by their poor and often biased documentation. New and adapted observation and documentation strategies, based for instance on the systematic collation and analysis of opportunistic data such as amateur videos, have to be implemented to enable a real progress in the science of extreme flash floods.