First, are you certain the challenge is broken? Usually if a challenge is doing something unexpected, it means you may need to re-read the briefing and approach it in a different way.
If you are really sure a challenge is broken or that you've found a bug, please do email into firstname.lastname@example.org with the challenge name and a detailed walkthrough of exactly how to re-create the problem. If, after an investigation, we determine the challenge is working as expected, we'll let you know. While you wait to hear from us, keep trying using a different approach (some of our challenges can be solved in a few different ways), and keep working on other challenges.