We know that to get a slave out of hung state most of the time is to reboot the machine. We can't reach the reboot step because we get blocked on trying to SIGKILL (I think) but that fails or it has already exited. Any thoughts on how to do this?
The ProcessAlreadyExited() error is a subcase of this bug, pretty sure that's already filed.
Let's say this got fixed in bug 626486. If it happens again we will have to revisit.