Get beside one of the blue walls that say "Subject cannot access memory at this time". Fight a guard in front of the wall and combo kill him. When he dies you will appear in the forbidden side of the wall. You can take a quick peek, but after a few seconds you will be teleported to your last checkpoint (as if you died).

If there is a viewpoint nearby the blue wall facing it with a hay bale on the other side of the wall, you can do the Leap Of Faith into the hay bale.