The other day I was playing and ARAM with a friend. He had Kalista and I had Karthus, and I joked that he should make me his Oathsworn and I would throw myself into the middle of the enemy team with my E... And well, he did, and things got a bit weird.
At some moment, I died in a teamfight, but my friend didn't notice that and he ulted me while I was on my passive. The thing is, I COULD use my part of his R and relocate myself with it even if I was with my passive on... I told him to not do that if he could avoid it just in case because we didn't know if it was or not a bug, but since my friend was so lost in the middle of the fights and focused on kiting, he did that one or two times more not noticing that I was already in my passive, and this happened every time.
A player told me (in the discussion I did in the Spanish EUW forum) it's not a bug and that Karthus doesn't count as "dead" while in his passive and so Kalista can R him and he can use his part of her R, but I don't know if what he was saying was correct or not (and nobody else replied with any useful info or something), and I wanted to make sure if it's a bug or not, and in case that it is, to report it.
So if anyone knows if it's a but or it's a legit thing, tell me, and if it's a bug, well, it's now reported xD
And, well, that's all!