At the time the incident occurred, Liberator was a new aircraft, not known to airmen. It was painted in unusual camouflage, and arrived unannounced, without IFF, taking a short cut over occupied France. Crew did not fire verey's of the day. Attacking pilots were confused, and send the aircraft down with one burst of fire.
As to blue on blue or friendly fire, American ones are best known, especially if they involved foreign aircraft (RAF, RAAF, RNZAF, SAAF) because they were considered international incidents. Nonetheless RAF downed quite a number of own aircraft, though it looks that several such incidents were either covered up or not identified as such. It becomes obvious only now, eg. Bader case.