Start of by placing a block with redstone on it. Then place another block diagonally (left one block, up one block/right one block, up one block) and place redstone on it. Finally, place a piston on top of the first redstone you just placed. You will see that the texture of the connection of the two redstone is shifted some pixels away.
EDIT by SuperGeniusZeb: It should be noted that functionally/server-side, the piston cuts off the wire, and the dust below the piston will power adjacent components like a normal plus-shaped dust (you can verify this by placing a redstone torch underneath the block the dust-that's-under-the-piston is placed on). The bug is that visually, the connected-upwards side texture appears, even though the dust isn't actually connected to anything. Also note that the piston cutting off redstone wire is intended - see