Happened to me with Manus. Had a homing crystal soulmass loaded and started running towards him, then he casted a spell the moment before my mass flew away. He died just before me, so "victory achieved" and then "you died" :)
I've had a power within kill when I ran out of heals on an early playthrough leading to a double kill. I think it was vs super smough bro. Also a curious thing happens if you die as a phantom but host/other phantom kills boss within the next couple seconds: you get the humanity (and medal as sun bro), and you come back to wherever you got summoned from, but all level enemies are reset.
I've always thought this had some good invader trolling possibilities as a host, but I'm too lazy to actually do anything with it. The downfall of course is you can be brought back anywhere, between 3 newly revived basilisks for example.