I just purchased a Phantom 2 V2 and have flown it 3-4 times with no problem. NO crashes...NO hard landings. Everything is fine. I put prop guards on before any of the flights...again the first 4 had no problem. Just went to fly it again and it was about 10ft in the air when the front silver prop flew off. I confirmed all props were in proper location. I also hand tightened all props before flight. Dislodged prop is not damaged and no obvious sign of damage to the motor. Now the motor studders...and goes in both directions just a few rotations at a time. Any ideas what's causing this?

sounds like the motor went fast 'backwards' and since they are self tightening when running fowards, that means they are self loosening when they are running backwards.  maybe bad ESC causing the motor to spin out of control backwards and 'unscrew' from the prop.  idk the interworkings of the ESC but just guessing.  but sounds like either the motor or esc has to be bad for that to happen.  did it crash?

The motor behavior you describe sounds like a blown / failed ESC (electronic speed controller).  There are four of them (one per motor).  I just finished dealing with this problem on my P2.  I won't rehash it all here, but there are issues with some of the v2.0 ESCs that shipped in some Phantom 2's.  DJI released a new version of the board that supposedly solves some of these problems.  

Search these forums or elsewhere for "DJI ESC failure" and read about it.   

I would contact DJI support if your P2 is still under warranty.  Otherwise, one board runs about $18-20, but if you're running v2.0's you'll want to replace all four.  Replacing one is relatively simple if you're comfortable with soldering.

gnixon2015 Posted at 2015-2-15 09:30
sounds like the motor went fast 'backwards' and since they are self tightening when running fowards, ...

Yes it crashed from about 10ft. I was starting to film a shot of my son...then throttled up and thats when it happened. Luckily it wasn't from 100 ft! No obvious damage

Same thing happens to me yesterday. I am shocked. Flied for fifteen minutes and it was perfect. I landed it ok and then decided to hover around a little bit more for practicing. Then one silver prop flied off and crashed from a few feet tall. Now one motor is not working properly. My landing gear is broken. God.

xtroar@gmail.co Posted at 2015-2-16 07:47
Same thing happens to me yesterday. I am shocked. Flied for fifteen minutes and it was perfect. I la ...

Yes, that's bad news!  I always give my props a really good tighten before flight, as a prop spinning at, I guess 4000rpm or so, has a fair bit of momentum going on, and a motor stall even for miliseconds will be enough to very quickly unscrew a prop, which still wants to be travelling at that speed.

I notice this has been an issue with the Inspire, so much so, that DJI have now developed prop locks. I wonder whether such a clip is being designed for the Phantom?

Cheers!
Rod