Buk M-1 missile vs. Flight MH17: What Went Wrong?

Almost no one thinks the shooting down of Malaysia Airlines Flight 17 was intentional. It’s hard to imagine an upside for Russia, Russian-backed separatists or Ukraine in blasting out of the sky an airliner full of civilians from nations not involved in the conflict. At the same time, the most capable, modern air-defense missile systems are not simple to operate. So what might have gone wrong that allowed a Buk-M1 missile to take down MH17?