The bug is very easy to spot: just stand on a block, press F3 & look down. You'll see that the z-coordinate for your feet and the z-coordinate for the block that you are standing on do not match. The z-coordinate for your feet will be 1 block greater than the one for the block you are standing on. This only happens with the z-coordinate, not the y- or x-coordinates. This bug makes using coordinates extremely confusing, because the z-coords on the F3 menu are offset by 1, where everything else is normal. It gets even weirder around ~ ~ 0, as what would normally be -1 becomes -0.