This happend when flying my 4x Mosquito on a D12-5 during the summer of '99. It happened instantly after the launch button was pressed and caused a fireball to fly about 100' up. Someone even asked me if it was a D12 because he "recognized that fireball". The motor case was never recovered so I don't know the lot number.
The damage took most of the motor mount and the hook. It also took out
two of the three TTW fin tabs and the panel between two fins. It also blew
the nose cone and tore out the shock cord. It never moved off the launch
rod. The rocket has been rebuilt but the only salvaged parts were
the fins.