In January 2003 there was a fatal AS 350 B2 accident in Canada. The pilot was very experienced but lost control.
The conclusion from the Transportation Safety Board of Canada (TSB):
Causes and Contributing Factors
1)After experiencing a hydraulic system failure, the helicopter departed controlled flight and crashed while manoeuvring for landing. The reason for the departure from controlled flight could not be determined.
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Full report :A03O0012
http://www.tsb.gc.ca/en/reports/air/...2/A03O0012.asp