Take the following diagram:
. E . X X . X X . X X . X X . . A .
Considering the Attacker has line of effect (like an arrow slit) to the Enemy. According to RC pg 219 on determining cover, lines along the edges of an obstacle aren't blocked. Therefore, the Attacker has only two lines blocked (the two corners on the left side of the enemy square) since the right side corners' lines are along the edges of the obstacle (with one on top of the other).
In that case the target will have only partial cover???
EDIT: For comparison, in the diagram situation below (still with an arrow slit between them) the target will definitely have superior cover.
. E . X X X X X X X X X X X X . A .
EDIT2: I increased the blocked terrain to be more visual, consider it a arrow slit tunnel.
EDIT3: To make it even clearer, assuming corners are numbered clockwise.
1 2 . E . 4 3
In the second case the lines to all corners are blocked (giving it superior cover). In the first case line to 1 and 4 are definetely blocked, and lines to 2 and 3 overlap, that would make it have partial cover by the rules, but both situations are pretty much alike unless cover is affect by "peeking on the side". Am I right in that thought?
EDIT 4: New diagrams to further clarify the matter:
Diagram 1 Diagram 2 . . . . | . . . . . | . .A. . . [E. .A. . . [E. . . . . . . . . . . | .
This | is a wall, and this [ is an arrow slit, they both sit on the edge of squares. I know they are assimetrical in relation to defense (meaning the Enemy has cover and the Attacker doesn't). The point I'm making is: in BOTH cases we have an enemy behind an arrow slit, but in the first case, since the wall ends after the arrow slit, the enemy would have ONLY partial cover, instead of superior cover that I expected.