Oh yes, I forgot one of my all time favorites in Super Mario Bros. 1. World 18-1, or I-1. I accessed VIA three Game-Genie codes in the SMB Cheats area of the site, two which I kept the same and the third which I took apart so it could be used to access other levels. What it is is a very, VERY glitched up version of World 3-3 that manages to still keep its stability. I have a DOC file that covers many of these "lost levels" in detail, but I have to find it. Another is World 9-1, a highly unstable underwater 6-2. There is also another highly unstable level that is an overworld 3-4, but I forget what World it is...