Hi Thomas,
I've also had this ("only" once, that is), but I could get that last part out of the sprocket. I think in my case it was caused by the set screw backing out, giving the mast a little play in the sprocket. Pumping the drive long enough this seems to be enough to break the mast. Do you have the sprocket opening warped a little? I had that. I believe in that case it is useless to buy a new mast, for that will probably break once as well.
As Matt indicates, the newer V2 sprockets (with the screw in mast system), solve all that. I replaced my sprockets and the masts, and this problem is gone. I used Roadrunners excellent manual:
viewtopic.php?f=60&t=12580. Check this for the details.
It may have been nice if this upgrade would be a free one for the turbo users. For I believe the set screw design is inadequate for the turbos. But that may be asking too much.
Good luck with the repair!