A while ago I put up a challenge for software testers. Here is the mission I used back then:
Regression Test Calculator
Test the regression test calculator for any flaws you can find. You might gain bonus points, if you can find out how the calculation is done. Another set of bonus points if you can come up with a better approach.
In the meantime I found out that Ajay Balamurugadas actually found the link to the website, and sent it to James and Michael. I think he deserves some special kudos for this.
These are the responses I received.