There is a race condition in STOP_FLUSH when a node joins the cluster.

JOINER sends JOIN_REQ to MASTER
MASTER does a flush on the existing members (does NOT include JOINER)
MASTER sends JOIN_RSP
MASTER sends STOP_FLUSH

JOINER receives JOIN_RSP
JOINER fetches state, sends START_FLUSH
JOINER receives STOP_FLUSH from MASTER (does not apply, since JOINER was not part of the original FLUSH)

onStopFlush never verifies that the current node was part of the FLUSH, and therefore is valid for the current node.
This STOP_FLUSH corrupts JOINER's FLUSH by resetting all the member variables (and probably unblocking as well).