Anyone who hasn't used this cross eye method on a spot the difference game needs to try it, it's a very interesting experience to see the way the differences are highlighted.
Agreed, I just tried it. Two good examples for this are the ones in the Wikipedia page [0]. The size in the page is maybe a bit large, if it requires a too large angle of eye crossing: hit Ctrl - a few times to zoom out.
Wow, it took me a bit of trying, but I was surprised at how clearly I could see the "compound" picture with the right amount of "cross" when the rest of the world was blurred. And at how well the differences stood out.