Rare, but not that unusual. of the 400+ Bloodhound Mk 1 and Mk 2's that were fired from Woomera and Aberporth between 1958 and 1986, at least 4 suffered boost motor explosions at some point during the first 2-3 seconds of flight. When they were phased out of service, the 1200 odd live boost motors that were going spare were taken to Orfordness (according to a bloke I know who was involved in the operation), bolted down and fired. According to my source two of the motors exploded instead of playing horizontal roman candle.