In the crucible on respawn, there's a chance to become a "ghost" relative to other players. It's happened twice now that I am unable to be seen by any players or damaged. In control I am still able to capture zones and kill people with grenades and rockets, but melee and bullets do nothing. It's varied in how long it lasts and I'm not sure how to recreate the bug, but if someone finds out it could be very detrimental to the atmosphere of the crucible.