I just played an intermediate AI bot game where the enemy Karthus became unkillable halfway through the match. Damage that would bring him to 0hp, would cause his health bar to disappear briefly, then reappear with 1 hp. No mater how much damage was done to him, Karthus would not die. After this happend the first time it happened every time.
I dont know what caused it or when it started exactly.