The Cats edition of the World's Most Difficult jigsaw puzzle is exactly as it sounds! The puzzle is double-sided. It has the same picture on both sides, offset 90 degrees from each other.
Having the same picture on both sides makes any puzzle hard, but there's more to the story. Jigsaw puzzles are cut in a press, stamped out like dough cut with a biscuit cutter. This makes a groove in the top of the puzzle and leaves the bottom flat. Any smart puzzle worker would easily pick up on this and be able to teel immediately which side is which, except that...
This is no ordinary puzzle! The puzzle is cut once from one side, cutting the verticle locks from the front, then flipping the puzzle over and cutting the horizontal locks from the back. There is no way to tell which side is which! Each piece must be compared on both sides for size, shape, colour and fit. This is very important. If the pieces don't for perfectly, you've got it wrong. If all this sounds confusing, just wait until you try it!
529 pieces - seems more like 4000 pieces.