On 31 October 1994, an ATR 72 exited controlled flight after a flap retraction when descending through 9000 feet was followed by autopilot disconnect and rapid and very large un-commanded roll inputs from which recovery, not within the scope of received crew training, was not achieved. The investigation found this roll upset had been due to a sudden and unexpected aileron hinge moment reversal after ice accretion on the upper wings aft of the leading edge pneumatic de-icing boots during earlier holding in icing conditions which had been - unknown to the crew - outside the icing certification envelope.