Happened to me last decemmber and this month in PvP Cyrodiil (PC-EU Sotha-Sil).
The enemy-players underneath were without HP but kept standing in the same spot, slowly regaining back their health, without us being able to harm them.
I threw Luminous Shards on both, no damage return numbers visible. Other players tried to harm them as well, no effect.
On the last one I could pull a Repentance, so he was considered dead.
Both stayed put at the same spot, but moved, (sheating weapon, reading map/book)
First player ran away after fully regained health, last player just disappeared after regaining health.
Anyone knows about a cheat that causes this?