Sure: we wrote a very simple retail trading system as a one-file Ruby app. It used a custom binary protocol. We had candidates reverse the binary protocol, implement enough of it to test the system, and find vulnerabilities in it.

We had a grading rubric for the challenge that was based on the kinds of things people found in the application. We evaluated performance on the challenge on a 1-5 score; depending on how you did on other challenges, a 3 would keep you in the game, and a 4-5 probably assured an in-person interview.