This happens when the player you were trying to kill is just unconscious, and exits the game (no respawn) before getting killed.

It happened to myself:
I was hit unconscious by a zombie, on the coast near Solnichniy, near a spawn point.
I exited to server browser, a few minutes later I rejoined the server and spawned near the place where the Z hit me.
My character was standing upright, Z was stilling hitting it. The character's head was turning around from side to side on each hit, as if the player is still alive and looks around.
I got rid of the Z and was able to loot the body. I hit it with a fireaxe but it just started bleeding.
15 minutes later, a friend joined the server and spawned at the same place. He discovered the glitched character, still standing. It did not disappear within 10 minutes like normal dead bodies do.