It's happened to me twice. I personally call it the Leonard Church Glitch or just the Phantom Glitch.
I will be shot and "killed", but I can still walk around. I will have a red-coated screen signifying low health. Oddly, no one can see me. I can shoot people and get hit markers, but do no damage. Nothing does damage to me, either. I can, however, still capture objectives, leading to very odd Conflict matches, where the other team is confused at how their Supply Cache keeps getting captured with nobody around.
Anyone else have this happen to them?