Blocks re-appear after being destroyed. I know this happens from lag, but it doesn't appear that way right now. It never happened to me on 1.7, and 1.8 is supposedly less laggy than 1.7. Again, blocks never re-appeared after being destroyed for me in 1.7 (except on some multiplayer servers), but in 1.8 pre-release, they keep re-appearing, on singleplayer. My settings are turned down, as well.