It happened to me and two randoms that invited me on that Mars strike mission. On the final boss area, someone tried running behind the pillar from the Cabal boss but he shot him and his ghost spawn is inside the pillar. No way to revive him, we managed to finish the boss off while that guy was AFK the whole time. There was a similar case with mines too. But this was during the beta. I was finding ways to glitch and stuff. XD