Glitched mobs are mobs that after killed, stays without a name tag and doesn't go away or be killed, but can't do any damage either. The reason behind this seems to be that this glitch happens rarely when the mob that spawns' targeting script doesn't work, and so stands in place while you kill it, doing no damage. Currently there is no way found to force this to happen. Also, there is no way of inflicting damage to the mob or dealing knockback.
The Solution to this problem is to give the mobs time when their targeting scripts don't work or just leaving that server. NOTE: This only works BEFORE the mob becomes glitched.
To fix the glitched version, you must lure it off the map. Sadly, the only floor that this works on is F7.