On October 7, 2008, Qantas Airlines Flight 72 from Changi Airport in Singapore to Perth Australia experienced a "sudden [uncommanded] nose down" event that resulted in a 650 foot loss of altitude, serious injuries to 13 passengers and 1 crew member, and minor injuries requiring medical treatment to 60 others.  This event was classified as an accident by authorities due to the extent and severity of the injuries.

The ATSB has determined the most likely cause of the accident was a defect in the software of the Northrop-Grumman (formerly Litton) manufactured Air-Data / Inertial-Reference Unit (ADIRU) which permitted the device to utilize data from a failed accelerometer within the device.  Data from the ADIRU fed other aircraft systems on the Airbus A330-300, resulting in false stall and overspeed warnings, loss of altitude indications on primary flight display, and abrupt action by the primary flight control computer. The costs that the manufacturer will incur for fixing the software and re-certification of the ADIRUs might easily exceed $2-Million. Airworthiness directives have been issued by aviation authorities.  

EASA's Emergency Airworthiness Directive is here.
The FAA's Airworthiness Directive is available here.  
The ATSB Preliminary investigation report is available here.

Here is what the news reported: