I was visiting Peenemunde launch site and museum a couple of months ago ( Place where Wernher von Braun designed, manufactured and tested the V2s in late 1930- early 40's.) There I learnt that nearly 1/3 of the rockets fired were exploding on site at the beginning , mostly during the refueling process, and that later in the war a large number of rockets were lost due to sabotage..
Amazing to see how little has changed 70 years later...