Glitch: In the beginning training missions (the white rooms). In the mission where you are trying to find a door and guards are walking around underneath you take out the guard in one of the buildings and stick him in the left hand corner (facing away from the door) then throw 1 or 2 grenades at him (this is the only time in the game this will happen). When they land on him and explode all the walls will be covered in red paint (presumably blood) even on the ceiling - this should not be happening.