Fun: Black Box Test Machines

Recently I found this site were some example programs are made available to
Black Box Test Machines by Workroom productions from James Lyndsay.

Quoted from his site: "The following freeware machines are testing puzzles - crosswords for testers. Open one up and play with it. Try to answer these three questions: What's it doing? What's it doing wrong? How can I be so sure?"

When you have some spare time you might take a look at those applications and try to answer the questions. It is also worthwhile to read his award winning paper: Adventures in Session-based Testing"