I've noticed this in both 13w42a and b. I'm not sure why this happens, but I think it might have something to do with the new block-highlighting system as seen below. The bug, however, appears to be an issue with the outline of each individual block. In the picture the grids look different than in actual game play. In game play, both the grid and the block-highlighter look that same. However, the grid patter only appears 6 blocks away from the player at all times (see "The grid lines move with the player" 1 and 2). As I said, I don't know what caused this, but when I looked it up I couldn't find the bug, except mildly similar bugs in older versions such as MC-7813. I apologize if I didn't look hard enough and this is another dup. but I really wanted this taken care of, it's massively distracting.