Are the stairs made cubes or blocks, or some sort of flat grid mesh? In my recent experiments with hair props, I've found that a flat "grid" mesh appears to result in the kind of a kind of one-sided object, where, for instance, you can't see the back of the hair (the inside, really) from the front, but turn the object around and there it is. To me, this looks like the backside of just such a mesh. I'm guessing that the invisible side is just showing the reversed normals of the front. Certainly, reversing the normals should work here. But, if you ARE using flat mesh grids, just start using cubes or whatever 3D primatives, and you should be able to avoid this problem.