The San Francisco team that won $50,000 in the DARPA Impossible Shredder Challenge has offered some insight into how they solved Puzzle #4.
The team used a combination manual/automated approach to solve the challenge puzzles. The first 3 were solved within the first 2 weeks of the challenge. After that, the team was stumped by puzzle #4. The difficulty was due to the fact that the words were irregularly spaced and written on unlined paper. By chance, one of the team members read an article about a little known government project that encouraged manufacturers of color laser printers to place tiny imperceptible yellow dots on printed pages so that the machine that printed them could be tracked. The team used a blue light filter and were able to see the dots. They credit those dots for giving them a road map of how to put the document together. A road map that ultimately led them to share in a $50,000 prize.
For more on the use of printer dots, read this article from USA Today: