DO YOU KNOW THIS MAN?
src: http://www.news.com.au/story/0,23599,22548669-2,00.html
The left part of the photo above is a part of an image posted on the Internet of somebody who has posted more than 200 images of him sexually abusing children.
Little did he know that the filtering algorithm he used (easily done by software like Adobe Photoshop) can be UNDONE essentially by applying the algorithm - in reverse. It's not a perfect recovery as you can see because the filter discards some of the original data in the filtering process, but it's good enough that you'd recognise the guy if you knew him.
Quite a crafty bit of detective work i thought!