It was human error for the fact that they were too stupid to figure out it was a passenger airliner. Comments have been stated that they "thought" it was an incoming missile. A huge difference in a missile and a passenger aircraft which is registered through the gov as well as flight scheduling. Also, there should have been better communications and all flights grounded during this hostile time frame.
They meant to shoot it down, so not an accident. Why in the world was it not confirmed to be a missile? They know the difference...