So have I - fun trick to play at a bar or wherever they have the (now defunct?) MegaTouch games. Although, when you ran into a dim or out of focus screen (the old ones used a CRT) it didn't always work that well. And you'd also find the bugs in the game where certain images didn't have the differences correctly marked!
I can't believe I've never used this trick on those stupid games! I've known about it for ages (good trick for visually diffing a bunch of hex on screen) but it never occurred to me to use it for those silly, silly games.