My Arducopter RC2 turned off all four motors at about 100' up today, and plunged to its death with no rotors spinning.
I believe there is a software bug or misguided feature at the root of it.
I was using GPS Hold to pin position, and the Arducopter was ascending. I brought the throttle stick down (while still in GPS Hold) to slow the ascent. I brought the stick all the way down. It was still a bit higher than I would have liked, so I switched GPS Hold Off and moved the throttle quickly to center/hover position.
This is when the motors turned off.
I can actually recreate this same scenario on the Configurator! In fact, any quick motion of the throttle stick from full down to mid-stick, while in flight, will kill all the motors and disarm.
Go try it and let me know if this is intentional/normal. It seems to auto-disarm if you bring the stick up quickly. That can't be good.
I have to get a new frame kit and a new motor/prop from Thailand now. I'm going to go look for the responsible bit of code and excise it.