The second screen of World 8 contains five special spaces that have a chance of pulling Mario into them for a very short level, upon which he's rewarded with an item.
Two of these spaces (the second and fourth ones) are unreachable through normal means, since Mario moves two spaces per press of the dpad. He'll always skip over them.
Say I game-genied Mario so that he only moves one space at a time. Would I ever encounter anything in the two unreachable slots?