We noticed that our signup metrics had dropped suddenly late last week, and needed to investigate why. We initially looked at our ad sources, but after finding that they didn't reflect the same drop, began looking at the signup process itself.
Our existing Conversion Funnel showed us that most of the dropped users were dropping out at one stage, so we used Paths From to show us all steps after that stage. Paths From showed us that users were taking a strange loop in the pages they were viewing, starting with a link on a page in the signup process. Reviewing the link, we found that it was misleading, and leading users in the wrong direction, preventing them from completing the signup process. Changed that link, and sign ups went back up!
Do you have other examples of debugging with Heap?