Let's try Caesium
Sorry, I thought this was reasonably well-known. See "Death" on this page.
The new thing I have spotted is the possibility of turning these "0% health at level exit" zombies into ghosts.
In the text-file in that zip, Lee Killough wrote:
I. Zombie Players
When the total health of a Doom marine and at least one voodoo doll (duplicate) is less than 100%, then the marine turns into a zombie when he is killed. He can still move around and operate switches, doors, etc., but he cannot shoot, and no weapon is shown, not even a fist. The health shown on the status bar is 0%, yet the marine is still "alive" in some sense.
After becoming a zombie, a marine can still pick up items, including weapons and health. However, the weapons are never shown again on the current level, except on the status bar.
When a marine is a zombie, any monsters that were already awake still pursue him and can hurt him, but other monsters are not awakened by seeing him until he picks up some health, even a 1% health bonus.
If there was 0% health at level exit, then the marine is a zombie on the next level (even if it's the same level, in the case of a "misplaced" secret exit). However, on the next level, he cannot pick up any items any more, which means he will forever remain at 0% and thus forever be invisible to other monsters, unless iddqd or idbeholds is used to make the health nonzero (but even if these cheats are used, he still cannot use weapons).
If there is any health during level exit, then the marine continues normally on the next level, with full faculties, including any extra ammo and health that might have been picked up.