Yep, I've had this before. It only seems to happen when two separate, planar polies (that don't share any verts) have a common edge. Cubes have separate polies that share edges but we dont see this effect, presumably because the polies involved aren't planar?! I'm not really sure why this makes a difference, TBH.
Other than trying to get rid of them by setting the CLS color to black, I don't know of any solution.
BTW, if your level is just going to be flat black as shown, I can't see why you have a seam at the place shown?
[edit] Just had a thought. You may be able to get rid of the effect by doing a 2 render pass of the level mesh: do a wireframe render before doing the regular render. Might work. :)
Other than trying to get rid of them by setting the CLS color to black, I don't know of any solution.
BTW, if your level is just going to be flat black as shown, I can't see why you have a seam at the place shown?
[edit] Just had a thought. You may be able to get rid of the effect by doing a 2 render pass of the level mesh: do a wireframe render before doing the regular render. Might work. :)