Sometimes when you walk, suddendly a chunk not far from you (but never the one where you walk on) briefly disappears and reappears. You can backtrack and make the chunk disappear again. If you stop in place, you can make appear and disappear the chunk just moving a bit or looking in different directions without moving.
I captured a short clip here: http://www.youtube.com/watch?v=ssrInbCsgw8