Apologies if this is a duplicate; I failed to find a ticket describing this bug.
When generating a new world in the newest MC snapshot (13w23a), I noticed strange behavior when loading chunks. Only the chunks immediately adjacent to the chunk I was standing in seemed to generate, along with some chunks on the edge of the radius of chunks which should have loaded. However, I was only able to see caves in the distant chunks, presumably because the in-between chunks were not visible. This bug was resolved after I changed the render distance in the settings.