One contributor to the A320 crash off Brazil in 2009 (Air France 447) was that the two pilots were making opposite inputs on their control sticks. The aircraft was in a stall, and therefore it was crucial to push the nose down, to regain airspeed. The instinctive human reaction (of untrained people) is to pull the nose up, since the airplane is falling. To oversimplify a long sequence of events drastically, pilot made the correct move, but the other pilot apparently panicked, and pulled back on his control stick. He continued to do this as they fell from 40,000 feet all the way to the Atlantic Ocean.
A new accident report says that the same thing happened in the crash of an Indonesia AirAsia Airbus A320, flight QZ8501, last year.