I think I mislead you regarding the transitions to down (when reachable) by referring to https://www.assembla.com/spaces/akka/tickets/1907
In the code I can't see that we disallow this transition, but we don't have any tests for it so it is not a recommended operation.
I will adjust the ticket.

Anyway, the diagram is currently not consistent. The transition from joining to down should be removed, or transitions should be added from all other states. I suggest that we remove joining->down until we have tested it.