Thanks for answer, Antoine!
I see what you mean, but I'm not sure that it's so extreme corner case. We are speaking about Python and it's easy to create an put an unpickable object into queue.
I apologize for the persistence, but I just want make this case more friendly to user. I spent plenty of time to debug it and I think it would be better to prevent someone to do the same things.
If it's a difficult to fix, we can at least write a note somewhere in multiprocessing module doc to describe this behavior.