On 23 December 2010 a Jetflite Bombardier Challenger 300 climbing out of Moscow in night IMC experienced a sudden in-flight pitch upset in which the three unrestrained passengers were injured, one seriously, as a result of an inappropriate pilot response to an annunciation of autopilot stabiliser trim malfunction. Despite extensive inspection, no root cause of this malfunction, which had been transitory, could be found. Crew QRH guidance in respect of the fault experienced was found to be unhelpful and crew knowledge of pitch trim - which could have eliminated any pitch disturbance - was deficient.