It happens on all the levels (at least on the 360) but more noticeable on The Library (Worst), 343 Guilty Spark (less worse), Truth and Reconciliation and Keyes (less worse), Assault on Control Room and Two Betrayals (some places), Pillar of Autumn, Halo, Silent Cartographer, and The Maw (Barely noticeable). What happens is the wall will have either one line (The Library for example) or multiple lines (343 Guilty Spark) one side light, the other dark (in case of one line) or different shades (in case of multiple lines). It's better to play it to see it. I also want to know if it happens on the original xbox because I don't remember if it did or to this extent.