Graphical glitch makes game unplayable

I generated a superflat world using "Overworld" preset, and I found a village nearby. The rooves were all dark and there were monsters spawning in those dark areas. When I rightclicked the blocks, the stairs ontop became visible. Then I reloaded the world to see if the stairs come back, and the whole world got scrambled. I recorded it, but the recording software got a hiccup due to the intense lag, and it didn't save audio nor AVI indices (not sure as to what are these). I attempted to upload the footage, though YouTube apparently does not like corrupted video files :\ (I might try to recover that, VLC can strangely play it)