I created a superflat world with just one layer of air using the superflat preset 3;air;1; and I can't move at all.I used 1.8 to create the world and I could move perfectly.In 15w34d,I can also see "holograms" of blocks at different areas which are non-existent.When I move towards these "holograms",the sky would darken,which happens when players are falling into the void.