Where:
Raduta Keep in the area of the tower with ghouls.
What:
If the ghouls or ghouls aggro you behind the locked door, and you leave the area back down the tower, they ghouls will somehow follow you down the transition, even if the gate/door they are behind is still locked.
I have not tried to recreate this with them following you through any transition beside the one leading to the bottom floor of the tower. PC I discovered it on can't do such things safely