Has a flight data recorder or cockpit voice recorder ever failed in an accident? Has there been a time either during an aviation accident or during normal flight where the flight data recorder or the cockpit voice recorder devices failed or the data was significantly corrupted?
As mentioned in the comment above, there is lot of aviation accidents with unrecovered flight recorders. List from wikipedia:
https://en.wikipedia.org/wiki/List_of_unrecovered_flight_recorders