Quote from TGregg:
A talks to C, and A decides he thinks it's going down as well, so he dumps it. Does the broker borrow from B to sell A's shares so C stays short? Or does C get nailed? I presume it's the former, but don't know for sure. . .
I think I read in my margin account agreement that the broker will try to borrow the stock from another account, and that will usually be possible, but in the unlikely event that he cannot find stock to borrow on your behalf, your short can be covered (I think they call it "called back from you") at any time without prior notice to you.