I was talking to zombie on IRC and he brought up a scenario with CPOWs that I hadn't thought of. If the child sends a CPOW to the parent and then the underlying object dies, what happens if the parent sends the CPOW back down to the child. Currently, we throw an exception deep inside some code called while receiving the message. However, no one ever catches that exception and so the message is silently dropped (except that we still leave the exception on the cx, which is also bad). A good way to deal with this would be to use a "dead object wrapper" in place of the CPOW. We already have code to deal with that, include Cu.isDeadWrapper.