For years and years, this compiler has assumed that by filling in 4 shaded squares, this will automatically leave another 12 to correspond with these, but if one of THEIR shaded squares falls on the
middle line either vertically or horizontally, then that will reduce the overall corresponding squares by 2, as there will only be one other
square to shade in, as opposed to the normal three for that centrally-placed square. They have NEVER corrected this occasional error, and ALWAYS assume that there will be 12 corresponding squares, when every 2 months or so, there is 10!! Hope this is clear!